You love watching your teen play her sport, but with every game she seems less and less interested in playing. If it's come to the point where your teen wants to quit sports all together, you may be worried that she won't get the exercise she needs, and won't reap other benefits such as social support and higher self-esteem. Those are all valid concerns, but don't get hung up on pushing your teen into something simply because you want it for her. Instead, encourage healthy activity in whatever way that works for her.
Praise your teen when you see her make a good play or do something noteworthy in her sport. Don't lay it on too thick; your teen will know when you're trying to butter her up and when your comments are genuine.
Sign her up for camps or trainings that can help her improve her skills. Sometimes the reason kids don't want to continue a sport is that they don't think they're good enough -- something the President's Council on Physical Fitness and Sports calls "perceptions of physical competence." To combat this, the President's Council recommends "creating an environment that will enhance children’s perceived competence."
Get your teen's friends involved. As child and teen counselor Uriah Gilford for the Help This Kid website explains, most teens are motivated by friends, freedom and fun, meaning that a little healthy peer pressure may be in order here. Ask one of your teen's teammates to talk to her about her concerns and let her know she's still wanted on the team.
Talk to your teen about other issues that may be contributing to her desire to quit. If she's having issues with other players, talk with her coach about ways to improve player relations. If she's having issues with the coach, look for other leagues or clubs that she can join.
Require your teen to explore and sign up for another sport -- even if it's not the sport that you want her to take part in. If she's really unmotivated to continue in one sport, your concern should not be forcing her to play that sport, but instead to get her doing something that will keep her healthy and active. This might include competitive sports such as soccer or lacrosse, but it can also include things like dance, rock climbing or joining a cycling club.