Thank you for being so brave to explain all the statistical reactions. I admire your honesty. I think it may be two things that are happening simultaneously.
I would begin by looking at the unfinished grief from your boyfriend’s death. I think working through that in therapy would be good. Additionally, the fact that you preselect partners that can’t commit and then wobble back-and-forth when the relationship fails suggests that you may want to explore this pattern of attachment in therapy as well. While I believe grieving plays a big part I think this other dynamic maybe be very important in understanding why you feel the way you do. I would encourage you to bring these two points: the unresolved grief and your attachment process into therapy. Since your profile showed you are at a university means you have access through the counseling center to highly qualified counselors. They are usually very proficient in helping people sort through these types of issues, and I think talking to them about this will help.