My mom and my friends say I should quit doing something [swimming, tennis, violin, honors classes], but I love all the things I do. What can I do?
First of all, it's great that you have so many diverse interests and that you have the passion to pursue them! However, as the old saying goes, there are only so many hours in a day — and sometimes people have to make tough decisions about what they can do, especially when they want to do so much.
Both your mom and your friends are right. You do need enough rest; otherwise, your health will suffer. You also need to make a couple of tough decisions about what you can realistically do in the time you have — which probably means that you'll have to stop doing a few things.
It sounds like you need to determine your priorities in terms of your future. If you want to go to college, then schoolwork is the most important thing, plus an extracurricular or two. If you think you can get a swimming or tennis scholarship to college, then you should focus on one of those as your extracurricular. If you plan to study music in college and become a concert violinist, then you need to de-emphasize the sports activities. You get the point; you have to consider what you really need to do for your future.
Keep in mind that you can always come back to things later; for instance, giving up violin practice now doesn't mean that you can't study violin later in life. Just know that you don't need to do everything now; you have plenty of time ahead of you.