Yes, you are overreacting. But its also normal to feel frustrated in a situation like this if you think the best option for your mental health is to cut her out and move on, then do so. You're 20 and have so much ahead of you. Spiraling about what might have been will only hurt more.
Relationship Advice
Sub for relationship advice and problems.
This is is not about you or how much money you have to spend. No one has enough to spend when they're a student. She's not committed to anyone and can do as she likes. Your feelings are natural, but they might be misplaced. If you're interested in her romantically, you could let her know in a way that doesn't put pressure on her and she will potentially let you know how she feels about you. If she doesn't reciprocate your feelings, bow out of your friendship with respect and charity for her and for yourself. You will have taken a great risk. Every risk you take helps you to grow as a man. Whatever she says in response, you haven't lost a thing, in fact, you may be liberating yourself to pursue something even better with someone else.
Yes, I agree that you're overreacting. I suggest you nip this in the bud ASAP, though, if you want to have healthy relationships.
Jealousy comes from insecurity. This torturous feeling will keep happening until you work on building your own self-confidence and learn healthy coping methods. If you can't see a therapist, I'm sure there are plenty of resources and shared experiences online that you can gain insight from the start with, at least.
I'm glad that you sense that this is bringing out some alarming feelings in you. It might be a good idea to back off until you address them, even though I know that'd be painful too. You really do NOT want to let that kind of shit fester, though.
This is also a great idea. Don't let yourself indulge i n brooding over this, as brooding is not attractive to women, not even if you're Batman. :) Self-examination and self-improvement are more likely to help attract the people you want into your circle. You already have honesty about yourself, which is a great first step!