It sounds like you have two problems, one is you're not as physically fit as you were, which could be down to diet and lack of exercise, you may want to watch what you eat, include fruits and proteins, and exercise and work out to increase your stamina and become more physically fit, breathing properly during sports can also reduce your fatigue level, through the mouth and steady even breaths. Secondly, because of this and other things, you have lost a little self esteem, which is very important i think, if you don't have faith and confidence in yourself, it can be hard for others to.. None of these is actually based on your ability to play. So I don't think you should quit, especially if you love the sport, Look at others getting more playing time as a motivator to work harder and fight for your spot on the team, you must be good to have a spot on the team in the first place, just try to work on getting physically fit to last for most of the game and build on your confidence. If you do this, I think you'll notice a change for the better. Good luck