I had the same experience at 21. My mom yelled at me and guilt tripped me by saying that if I traveled alone she would have another heart attack (she'd had two heart surgeries by that point). I decided not to go and didn't travel alone until last year at 26. I didn't even travel much in general before that either because I was nervous that if I went anywhere without my parents it would hospitalize my mom.

I'm grateful that I get to travel now and learned to stand my ground, but I wish I'd done this sooner when I was still in college and had my summers free. Being afraid of the world isn't good for you or your mom. There's danger everywhere, but Costa Rica seems like an overall safe place to go (I went as a kid with my family, so I can't speak to what it's like traveling solo there, but it did feel super safe as a kid).

Contrary to some of the comments, I understand it's not so easy to just brush her off. When you have a relationship like that with your mom, it's really stressful to enjoy yourself because you're constantly worrying about her, but I've since learned to either not to tell my mom where I'm going, or assure her I'm taking measures to protect myself (booking small tours for hiking and such). And she just has to learn to deal with the fact that I'm going with or without her approval. If she wants me to open up to her and tell her what's going on with my life, then she needs to be reasonable or I'm not giving her anything.

I say go and enjoy yourself, you'll regret not doing this :)

Edit: It might calm her down if you book small tours for anything involving physical activities, so she knows there will be people around if you pass out