its hard to say, I mean there is little context. is this your first serious girlfriend?

it can be a lot of things. your best bet is to analyze yourself and if possible get a mental health professiinal. obviously thats obvious so I will share what I observed in friends and what I think.....

so there are lots of things. but some people I know obsess over an ex because its a refelction of their lack of confidence in themselves. I know this is obvious that most things come from lack of confidence in yourself, but ime, people who became resentful or angry at someone who dumped them, often had lots of self hating thoughts and havent dealt or at least manage their insecurities. most people who were confident in themselves, when they got dumped, just understood, cried a bit and moved on in good terms. these confidnt ppl then went and dated new ppl when they felt better.

however. the people who were insanely jealous and obsessive were those who you least expect it. anxious, sweet, soft-spoken guys. they didnt have that confidence in themselves to get new girlfriends and it seemed like a big deal to them if a girl dumped them. itt seemed like the end of the world.

in addition, chances are these anxious-sweet-ppl also tend to carry a lot of emotional baggage. some of these guys I knew had a critical mother who often berated them, so if a girl rejected them, they displaced a lot of anger towards these women. he thought they looked down on him when really rejection is totally normal paet of life. I mean there are million anecdotes I could write, but these are ppl who stood out to me.

I hope you dont mind me saying this, but try practicing compassion to yourself and others. you are thinking about yourself a lot, and this is understandable since we all think.of ourselves, but if you truly care, then you would want her to be happy and have her own autonomy in her choices.

I think just seeing that you dont have much control over the actions of others and how things turn out helps. I found some of the worst breakups my friends experienced were ones where they often fantasized about marriage or considered having kids when they dated only for 2-3 years only with their partner. they were thinking and building a vision of what could be, hence when the breakup came, it was intense. things change, people change, you change, emotions change (one minute ago you were crazy bf now you are calm and seeking help) .

point is, you can start off with why you feel this is so important to you that it drove you to the point of your crazy moment? then work to fixing the way you view things that might be contributing.to this.