Needs quick answers to this?

This is a gross question, but how do I force my poo to come out? I go for a poo everynight but very little is coming out. Sometimes I open my bum a bit more to see if any more will come out. I haven't been able to do a good poo for 3 weeks. I'm in a wheelchair so it's hard for my bowels to work because I'm always sitting down. When I finish my poo, I always feel alot still in my bum. I don't want to go to a doctor at all.I just want to get this poo out. I know that weight can cause this. I'm 9 stone. Do I need to lose any weight. Please give me serious answers. How can I force my poo out without going to a doctor?