Unless the shop has a returns policy, then you cannot return it after a month unless it is faulty, not as described or not fit for the purpose.
Did you tell the salesperson about your RSI and did you buy that particular mattress on the advice of the salesperson? If so, you MAY have an argument for it not being fit for the purpose you bought it, but frankly it is unlikely, especially as you have allowed a month to lapse.
I suggest you contact the shop and tell them you have given the mattress a fair trial but it really is unsuitable for your needs. See what they say.
Some companies do sell mattresses on a return if not satisfied basis, but that is not the norm.