Too soft and now have back problems (herniated disc, etc)

I was talked into getting this mattress by the sales associate and the "sleep test" at the store I bought it. Ultimately I kept saying I like a firmer mattress. I did receive my new bed frame until nearly 60 days into my return policy so could only sleep on my bed for about 30 days before the return policy was obsolete...which wasn't enough time for me to know that the mattress is terrible for my back. I have had back issues every since getting the mattress and now have to buy a new mattress because of it. I will surely not be going back to Sit N Sleep for their "sleep test" again or to buy my new mattress. I should have stuck to what I know.