I have been playing for two, going on three years, now. I love to play and most likely would play all day if it were up to me! But Ever since I have started playing, I have had this same problem, a most annoying problem. My lips start to fail to stay firm about 30-45 after playing or if I play 3-5 songs straight, like in a competition. But it'd starting to get out of hand. Other people can play all day, so I am just wondering if I am doing anything wrong playing...

you really do just need to keep practising. everyone has a point where their lip starts to go. if i have a concert, and im in rehersals all day, by the end of the concert i wouldnt be able to play another note. so yeah, you just need to train your lips into being able to play for longer, thats all. and make sure you're not puffing your cheeks out when your playing !

Hehe, when I first started playing, I didn't fold my lip over my bottom teeth and it came out bad, so when I was told the right way I would firm up and my cheeks would puff out. It took my a while to learn to play without doing that.