Poor Product

Good day

I have purchased a Serta Richmond bed from The Bed Store in Springfield last year July, within 3 months or so the bed was very uncomfortable and I could see the bed was sinking and could feel the springs coming through the bed which was piercing my side and back. I called and the sales staff said that it is normal for the bed to sink and I should rotate the mattress which I have been doing on a regular basis. I have now contacted the store again and was asked to email them all the details which I did. They then came back to me advising they will not give me back my money or replace the product but will repair it. Who will now be responsible for my discomfort and back injury? How can I now sleep on this for the years to come?