I'm a senior in high school now, but I swim competitively and we practice six days a week. I also play quizbowl and Science Bowl, and I'm on our student council. I spend some time on biology olympiad, chem olympiad, and programming competitions throughout the year, and I did biology research for the past two summers. I've been doing most of these things for several years, and I balance it by being mediocre at all of them. :^)
In general, I would say just try to develop a love for all the activities you do. When you really love doing something you make the time for it without needing to force yourself to. Conversely, if there are activities you do that you've given a chance but just don't enjoy as much, I would honestly consider dropping them. As an example, during my freshman year, I was on speech and debate but ended up quitting after six months, not because I hated it, but because I liked scioly so much more and I didn't want to divert time away from something I cared a lot more about. I quit doing competition math and physics olympiad in sophomore year for similar reasons (also I sucked at them, but that is unrelated ;P).
Oh, by the way, you should definitely try to work out some kind of arrangement so that you aren't missing every scioly practice for other stuff haha. If that's not possible, let your partners and coach know what's going in your life and your schedule. In my experience, people are generally pretty understanding, as long as you communicate with them regularly and keep them posted. I know things are stressful, and I wish I knew how to make it go away, but I think if you really care, sometimes you just gotta stick it out. Best of luck~<3