Hi Mary.S,
Welcome to the Mattress Forum! 
I’m sorry your Serta mattress didn’t work out well for you, but Serta, like many of the larger brands, tends to use lower quality and less durable foams in their mattresses (see below), so it’s not a surprise that your mattress is already not performing well.
Actually, it tends to be the opposite, with more brands choosing to go with lower density memory foams, which are less durable, which is why it is so important that as a consumer you become educated about the componentry within a mattress and eschew any product that uses lower quality materials, or products where you can’t find out the information of what is inside.
I wouldn’t focus much on the marketing terminology of terms like “hybrid”, which has become so watered-down to be almost meaningless. If you like the idea/feel of using an innerspring unit for the support core, there’s certainly nothing wrong with that. And latex is a very durable and resilient material and is a good choice as far as quality is concerned.
While I wasn’t there during your visit, I doubt that the salesman actually “yelled” at you, but was more than likely providing advice to not attempt to configure your own product, which would be sage advice. Unless you are extremely experienced with mattress construction and design (which would be very few people), your best starting point in a configurable mattress system would be layerings that have already been designed and tested by a knowledgeable retailer or manufacturer (and I do think highly of the advice that Jamestown provides). You could always fine tune from there after you’ve had the product in your home and acquired a point of reference, should you desire.
I’m guessing you meant Beautyrest here, which is a brand I would recommend avoiding (see below). When testing out a mattress in a showroom, you can’t feel quality or durability, which can only be determined through knowledge of what is inside of the mattress.
I don’t know where you are reading that but it’s an inaccurate statement, as most mattresses sold are still innerspring in their support unit and many people still have a preference for this type of design. The key is to find a product using quality componentry, which I’ll explain how to do below.
Overall, I’d suggest that you press the "reset’ button on how you’re shopping for a mattress. And while I can certainly help with “how” to choose … It’s not possible to make specific suggestions or recommendations for either a mattress, manufacturers/retailers, or combinations of materials or components because the first “rule” of mattress shopping is to always remember that you are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula or for anyone to be able to predict or make a specific suggestion or recommendation about which mattress or combination of materials and components or which type of mattress would be the best “match” for you in terms of “comfort”, firmness, or PPP or how a mattress will “feel” to you or compare to another mattress based on specs (either yours or a mattress), sleeping positions, health conditions, or “theory at a distance” that can possibly be more reliable than your own careful testing (hopefully using the testing guidelines in step 4 of the tutorial) or your own personal sleeping experience (see mattress firmness/comfort levels in post #2 here).
I’m not sure what you’ve read since you found the site but just in case you haven’t read it yet … the first place to start your research is the mattress shopping tutorial here which includes all the basic information, steps, and guidelines that can help you make the best possible choice … and perhaps more importantly know how and why to avoid the worst ones.
Two of the most important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of “comfort”, firmness, and PPP that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists based on all the parts of your personal value equation that are most important to you (including the price of course and the options you have available after a purchase if your choice doesn’t turn out as well as you hoped for).
While again nobody can speak to how any specific mattress will “feel” for someone else or whether it will be a good “match” in terms of comfort, firmness, and PPP because this is too subjective and relative to different body types, sleeping positions, and individual preferences, sensitivities, and circumstances and you are the only one that can feel what you feel on a mattress … outside of PPP (which is the most important part of “value”), the next most important part of the value of a mattress purchase is durability which is all about how long you will sleep well on a mattress. This is the part of your research that you can’t see or “feel” and assessing the durability and useful life of a mattress depends on knowing the specifics of its construction and the type and quality of the materials inside it regardless of the name of the manufacturer on the label or how a mattress feels in a showroom or when it is relatively new so I would always make sure that you find out the information listed here so you can compare the quality of the materials and components to the durability guidelines here to make sure there are no lower quality materials or weak links in a mattress that would be a cause for concern relative to the durability and useful life of a mattress before making any purchase.
The major brands such as Sealy/Stearns & Foster, Simmons, and Serta all tend to use lower quality and less durable materials in their mattresses than most of their smaller competitors that will tend to soften or break down prematurely relative to the price you pay which is why I would generally suggest avoiding all of them completely (along with the major retailers that focus on them as well) regardless of how they may feel in a showroom along with any mattress where you aren’t able to find out the type and quality/durability of the materials inside it (see the guidelines here along with post #3 here and post #12 here and post #404 here).
Regarding reviews, I would tell you to ignore them. While other people’s comments about the knowledge and service of a particular business can certainly be very helpful … I would always keep in mind that you are the only one that can feel what you feel on a mattress and I would be cautious about using anyone else’s suggestions, experiences or reviews on a specific mattress (either positive or negative) or review sites in general as a reliable source of information or guidance about how you will feel on the same mattress or how suitable or how durable a mattress may be for you. In many if not most cases they can be more misleading than helpful because a mattress that would be a perfect choice for one person or even a larger group of people in terms of comfort, firmness, and PPP may be completely unsuitable for someone else to sleep on (even if they are in a similar weight range). In other words … reviews or other people’s experiences in general won’t tell you much if anything about the suitability, quality, durability, or “value” of a mattress for any particular person (see post #13 here).
While this is a bit of information to read through, if you follow the steps outlined in the mattress shopping tutorial you will be in a much better position to make a more educated choice for your next mattress, and know what to avoid.
I hope that’s helpful to you.
Phoenix