Been reading music theory about proper posture during playing

I have been sitting slouched over on my bed playing for the past few weeks. So I have set up a chair and foot rest and music stand so I am sitting properly when I play now. Oh my it is so hard to do. My back and left arm get so sore and tired. I don't know if I'm going to be able to do this I can manage 15 minutes at the best but I usually play 30 minutes in 3 sessions. This is hard. Anyway I will give it a shot and see how it goes but it's not easy. It's cold in here today I'm tired as well so that could be part of it. I think I might have a nap.