You are not being selfish. If anything, she is being selfish for pressuring you. If you become commited to something without being 100% sure that its the right thing for you, then that isn't being fair to either of you. I hope that you can become better mentally and eventually be okay with moving to that step but don't do it from being pressured.

My ex used to pressure me to get married so after being together over 3 years I finally said yes even though I knew I wasn't ready. And looking back, thats what officially killed out relationship. He was pressuring me and I wasn't ready so getting engaged really put added pressure on our relationship that wasn't needed. So after feeling insecure about the whole thing and knowing it wasn't the right decision that caused us to fight to the point that we broke up and I think if we had never gotten engaged, we would have been a lot better off.

My current boyfriend is about 4 years older than me so he is much closer to being ready to settle down and have kids but he understands that I'm not ready yet. I told him that I didn't want to talk about it until I was graduated from college and accepted into grad school and got my career going a little. He is totally fine with it. I think you should have a talk about why she is so adamant about getting engaged and finding out if its because she wants to be with you or if she just wants to get married. I have a lot of friends that got married just because they wanted the wedding and not because of the person and they are miserable after less than a year. You need to do whats best for you.