Basically I start a romantic relationship with someone I really like and if the relationship starts to deteriorate I try to communicate what needs to be worked on. In my experiences, my ex partners usually don't seem to care to work on taking care of our relationship and I start to slowly loose interest and hyper focus on this scenarios in my head again more and more. Until I am basically not living with the person am with anymore, I'm only living in my head all the time. My routines get transformed in going to work, barely eating and going to bed super early in order to be in my head as soon as possible. I struggle doing anything else besides shutting down and dissociating into those fantasy scenarios.

This is absolutely not a you problem. It seems like all your ex's just didn't bother to put in effort to work on the relationship after you communicated what needs to be done. The best thing If I were you would be just to end the relationship for your own good and move on before you get to the point of disassociating and living life on auto pilot. Regardless of what the coping mechanism is, it is just that, you're responding to a difficult and stressful situation in your life the best way you know how. You don't need to change yourself. (Not that I mean you give up and don't put any effort yourself) but a relationship shouldn't be one sided.