I just broke up with my girlfriend of about a year because she wanted

I just broke up with my girlfriend of about a year because she wanted to step back in the relationship and have the possibility of dating other people. I am a 34-year-old man who is set in his career and she is 25 years old and just finishing up grad school.
When she first told me how she felt, I was taken aback. The first night I was fine, but I thought about it more and I just felt it was the beginning of the end, which is why I broke up with her. We had broken up once, a year ago, when she went back to her ex-boyfriend. During the relationship we had our ups and downs, but I thought we were happy. Whenever she doubted the relationship, we communicated what we wanted and we were good. Even though I thought we were happy, we weren't having sex as often as before and I chalked it up to her being tired because of school, work, and her addiction to over-exercising.
We still go to the same gym and sometimes run into each other. I always think about what could have been since we did talk about marriage and kids. We even glanced at wedding rings. Was I selfish in not giving into her request or did I make the right decision?