Too Firm Not as Advertised

We spent hours on this site and others comparing mattresses. We wanted a firm mattress and didn't want memory foam. Mattresses like Bamboo we tried in the showroom and they generally felt too soft and unsupportive. We are both heavy and the "firm" mattress we were sleeping on was so soft after six month it felt like we were trying to get out of a boat every morning. So, the reviews for the Brentwood being firm seemed good for our needs. I felt confident purchasing it because the reviews said it was very firm but Brentwood advertise this mattress as Medium Firm. My partner is a back sleeper and is happy with it. I'm a side sleeper. I wake up all night long with numb arms and fingers and pain in my shoulders. I wake up every morning with chronic pain in my hips shoulders and sometimes even my legs. I wrote to customer service immediately and they said well that's our firmest mattress, have 10% off our mattress topper. Im so angry that I have to spend $600 on a mattress topper to make my mattress work. I feel like this mattress is misrepresented on the company site. Knowing what I know now, I would only buy this mattress if I was under 100lbs.