Hi, I totally understand the overthinking and obsession with the relationship, and thinking whether you should stay or go.
I was with my exDH for 21 years, and for the last few years of our marriage, it was all I could think about. I got books about how to decide whether to stay or leave, had counselling, spoke to friends, posted online. I was so stressed and unhappy, but also felt paralysed.
I finally told him that we needed to separate and he moved out 6 months ago. I have been going through a lot of pain and grief, and I think this is what was probably stopping me from breaking up with him (I didn't want to face those feelings). Lots of stuff coming up about feeling abandoned and alone. My dad moved out of the family home when I was a teen and didn't stay in contact, so I think that affected me hugely and was partly why I wanted the relationship with my ex to work at all costs. Grieving the loss of the family unit - we have 2 dc. And really missing having a man in my life who loves me - ironically I didn't have that even when my ex was here, but it's almost like I couldn't feel the feelings until I was apart from him.
But on a positive note, I am also feeling a lot less stressed, I really enjoy having my own space and autonomy - it's lovely! The house is so much more peaceful and relaxed without him here. So although it has been so hard at times, I think it is simply a process of grieving, and if you had any childhood trauma re a father figure or relationships, the break up, and decision to break up may be harder.
Re how you will finally decide whether or not to leave - I think when or if it's the right time, it will just happen. I got to the point when I was in such mental, emotional and physical pain from him being in the house that I couldn't carry on like that any more. But it took a long time to get there, and I really get it can be a drawn out process.