Hey. First of all, I want to say a huge WELL DONE for finally making your decision about quitting your club team. That surely took a lot of courage to decide and I for one think you made the right decision.
I think it's good that you've decided to postpone your decision about swimming to the autumn. What should you do, vis a vis swimming? Personally, I think you should just leave it till then. You've decided that you're not going back until then, if you ever do, so why worry about it? Right now, you've said that you don't see a reason to go back, so don't even think about it. Cross that bridge when you come to it: for the moment, you can just enjoy your summer league practices and forget about club team.
It is absolutely not too late to be good at something because everyone is good at something! I know you said that you feel like swimming is your identity, but I'm sure that if you look into the other areas of your life, there's other things that you are good at. Nobody can be good at everything, but everyone is good at lots of things.
In fact, there's a good chance that you'd be really good at crew, because as a swimmer, and with the amount of exercise you're doing that isn't swimming, you must have a lot of muscle. That probably puts you ahead of a lot of other beginners.
I think that if crew is something you think you'd really enjoy, then you should go for it. However, remember what you've learned from swimming. You obviously try to put a lot of pressure on yourself, so maybe you could try to make sure you don't go for anything too competitive and try and keep it to a hobby rather than making it your life. Crew is a team sport (to my knowledge), so maybe you could try to get the other people in your team to help you if you start to feel anxious like you did about swim practice?
Going back to what you got out of swimming, I think you need to start thinking about it less in terms of black and white goals and more in terms of personal experience. It must have taken a lot of dedication to do all that work and get to be so good: there's lots of people who would never even have tried. I'm sure that you've also learned a lot about courage and determination and about how we sometimes have to make hard decisions because of your feelings before you decided to quit. It's certain that you've developed some great exercise habits for life, not to mention acquiring a lifelong skill that will always come in useful. So what if you weren't world champion? Only one person can be the best, and it really doesn't matter if it's not you.
You should try and find other ways of defining yourself other than 'a swimmer'. What qualities do you have, as a person? Are you sarcastic? Flirty? Funny? Serious? What's your style? What's your family like? Where are you from? Online, you've always seemed to me like a very bright and bubbly person - are you like that in real life too? We are all made up of thousands of different attributes: there's no point in defining yourself by one of them. I'm sure that once you're used to not swimming so much, you'll start to see the other sides of your character start to shine.
Will you gain weight? That's one area where I have to admit that I'm clueless. It's possible that you'll lose weight, if you lose muscle, but it's also possible that you'll gain weight, if you gain fat, and I really don't know how to determine which it would be. I advise you to talk to your doctor or nutritionist; they will be able to give you a good answer that will be far more reliable than anything I could give you.
Once again, I just want to say that you are coping absolutely amazingly with everything that's going on in your life. There you go, there's a character attribute for you: you're an amazing and remarkable person.
-Fern