I am told I have commitment issues because I can’t make up my mind about my relationship. My girlfriend wants me to talk about the future, and I want to, but the thought of saying something terrifies me because I have never been able to think about marriage. I know she expects me to propose, and we have been together for three years, but the thought of being with one person for the rest of my life is hard for me to imagine. She says she will break up with me if I can’t promise her a future and I am angry as well as scared because this feels like I am being forced. If she goes, I don’t know if I can manage, but I also know that I can’t be coerced into settling down. I feel trapped and don’t know what to do. Please help me come up with a solution.
This is what wanting to have your cake and eating it too means. You refuse to commit and think you’re being forced but aren’t sure about living without your girlfriend. If she wants to know what the future is like after three years of being with you, it’s a perfectly reasonable request. If you don’t have a response for her, and can’t come up with one, it’s obvious that these differences cannot be reconciled. Why should she be with you if you can’t give her what she wants? Be honest with her and accept her decision because there is no common goal here. There is no solution.

I’m struggling to accept that I am not my girlfriend’s first partner. She’s my first, and I feel like I’m not as important to her as she is to me. How should I approach this?
You should approach this as an adult by accepting that she is allowed to have a past. She was with someone but is now with you, and if you can’t accept that or appreciate it, it’s your problem alone, not hers to manage.