Hi rambo,
Unfortunately you are in the same position as many others that have purchased a major brand mattress and this one of the “side effects” of a mattress that uses too much lower quality and less durable materials in the comfort layers. While they may feel great in a showroom … they can quickly develop either visible or “virtual” impressions where the mattress sags under the heavier parts of your body and you lose the comfort and support that was the reason you purchased the mattress in the first place.
If your impressions are still visible when you have been off the mattress for a while and they are deeper than the warranty exclusion then you may qualify for a warranty exchange (it would need to have visible impressions that are more than 75" deep) but if they are only virtual impressions (which are more common) where the sagging only happens when you are sleeping on the mattress then there are few good options available to you although there are a few suggestions in post #4 here that may be helpful.
Unfortunately a mattress pad or a topper will likely only be a temporary or partial solution at best because it will just follow the sags in the mattress underneath it and at worst it may even make the issues worse.
If you do decide to buy a new mattress then I would make sure you read the mattress shopping tutorial here which has all the basic information, steps, and guidelines that can help you make the best possible choices … and perhaps more importantly know how and why to avoid the worst ones so you can make a more durable choice and don’t have to go through the same thing all over again.
There is also some information in post #3 here and the posts it links to about heavier body types that will be helpful as well because higher quality and more durable materials in a mattress (in the upper layers especially) will be even more important than for those who have lighter body types than you do.
Phoenix