It's truly messed up but yes, you're right, many women experience non-consensual groping or touch at some point in their lives. Your experience is valid and just because it doesn't seem as "bad" as what others go through doesn't mean it hasn't deeply affected you. I, too, would suggest you stop drawing comparisons. The things that cause us pain, cause us pain. This hurt you and it's not right.

I can't give you one sure-all way to make sure this doesn't happen. I remember mosh pits as a teen and that's kind of just what happens when you're in one. It's messed up but it's hard to pinpoint who is groping you and so you cannot really be assertive in setting boundaries/telling someone what they did was not okay. I think if you want to be in the mosh pit you may have to assess the risk that someone will grope you. I know it's horrible to say that, but with so many bodies all thrashing in one place you just can't single someone out for the most part. If it happens I would suggest taking a break from the mosh pit like you did the first time and seeking support from your friends, or maybe on here. Even though that's kind of just the "way" or things it doesn't make it okay and you deserve to feel safe and supported.

As for the gym, if you feel comfortable I think you can be more direct. You are a minor and you could definitely get the staff of the gym involved if you needed to. An adult male should not be taking pictures of a minor at the gym (although really no one should take pictures of anyone at the gym). You can tell him to stop and erase the pictures, or to please stop interfering with your workout. I think the idea of working out with a friend or taking a class is a good one, too. Working out in groups can be even more motivating and fun. If all else fails, consult the staff. You deserve to have a safe space to workout.

Ideally the world would be a safe place for women but it's often not, not in our current climate. But progress is being made. People are becoming more aware, more informed, and stronger advocates against this sort of casual touching behavior. You always have a right to speak up if you feel safe, and talk to your friends. They can have your back, too.