Bare with me, I don't usually respond in this much detail. First I wanted to say that acknowledging your fears like this, it diffuses some of the power they have over you. It's something you should be proud of, I've only recently been able to admit my fears and some of them I still struggle to understand, identify and accept.

As for existential anxiety, to solve it I think it's important you assign meaning to at least some of life. I find that if I read inspiring and wise words, I remind myself of the value of wisdom and kindness and it really is a solid foundation for everything in life. I pray daily, not to god, but just as a sort of meditation (something I learned from Buddhism), it helps me. Prayer helps keep your heart in the right place. Loving kindness meditation might be something that would help you. There is some guided ones on youtube.

Lets not sugar coat it, you've been through a very traumatic experience, for almost anyone. The fact that you've made it through it and moved on as much as you have is impressive. It could probably best be addressed through counseling. Basically you want to get to a point where you can accept what happened, and remember it at times, without avoiding it completely, or focusing on it excessively.

I do think you should talk to a doctor and get back on medication. If you want to try a new one, you could always do that. Medication is trial and error, and they often do have side effects which are hard to live with, but I find them to be worth the trade. For me, I take mine to have less anxiety, and also to not hear sounds at bed (that aren't there), and they help with obsessions. I take gabapentin and olanzapine.

I think with your girlfriend, you just need to be honest and open with her. You might feel so on guard around her because there are things your not saying. Maybe ask her how going to college will affect your relationship?

It sounds like you don't trust her, because of what she did. Maybe even say that you want to trust her but you are having difficulty with it? There's always a risk of relationships ending, but part of the effort that causes relationships to succeed is being open with one another.

As for defending your points of view and trying to think of arguments. One thing they teach in Buddhism is to not cling to views. Why? Well the number one reason is that it causes a lot of anxious thought, and tense arguments. Also, your views may evolve, what was true for you at one time might change. Also, you literally can't control other peoples thoughts and opinions. The best chance you have at convincing anyone of something, is you can do it in a calm, collected and open manner.

All that being said, nothing wrong with your opinions and views! You should have them! Just maybe having a different attitude, one that is less defensive or offensive would be helpful in alleviating some of your anxiety about it.

Anyways, I wish you the best! Feel free to correct me if need be.