Hi Idael,
That’s terrible that you’ve had a poor experience with 2 Sealy mattresses in a row from Mattress Firm! At your low BMI, there’s no reason the foam should be developing a noticeable sag in a few months. I took a look at the Sealy Opportune II:
13"
ComfortLoft Knit Cover
SealySupport Gel Foam
Response Pro HD encased coils
The lack of info on density of any of the foams causes concern; just to compare, we suggest any foam in a potential mattress has no more than an inch or so of lower grade foams (for memory foam no lower than 4 lbs/cuft for normal range weights, and 5lbs/cuft for higher range weights) and if polyfoam is used, with at least 1.8 lbs/cuft density, just to ensure you have support. Again, with your low BMI you should not have noticeable sag in the mattress in such a short period of time.
Keep in mind any mattress you choose will be based on your Stats (height, BMI, sleeping position(s) and any underlying health conditions), and your PPP (Posture & alignment, Pressure relief, and Personal preferences). You may want to take a look at the Mattress Specifications You Need To Know and see how they compare to the Mattress Durability Guidelines. With your lower BMI, any sufficiently dense quality memory or gel memory foam should be durable enough to retain its comfort level for quite some time.
You also might want to read our Mattress Shopping Tutorial to avoid the same pitfalls with mattress retailers in the future.
As for solutions…you may want to see if you can sell or donate the mattress locally if you aren’t able to comfortably sleep on it; we also have Sell/Buy and Donate/Recycle threads here on the forum. It’s possible you could use a mattress topper to provide a more supportive top comfort level - although depending on the severity of the sag, this may not be effective, or at best provide a ‘band aid’ solution.
Finally, if you are considering online ordering, you may want to review the offerings of the Trusted Members of the site, who all offer quality sleep solutions with transparency in their materials and components, excellent customer support and generous exchange and return policies.
Sorry you’ve had 2 ‘dud’ mattresses in a row, but at least going forward you can have more awareness of what to look for and avoid. Remember, the best consumer is an informed consumer!
~ Basilio