I read your other comments and I share some of your qualities like avoidant attachment for one.

Even though I sometimes ignore this, I do truly believe it: if the relationship was meant to be, then you wouldn't have had any doubts.

I've been in relationships where I had no doubts at the beginning. It happens. Those are the good ones. One of mine lasted 9 years.

Then I've dated people where I had doubts but really wanted to make it work, and it never ends well.

You're likely to feel some questioning or regret whenever breaking up with someone over some vague doubts because it's not something obvious like being cheated on. It's tricky but important to realize the doubts will probably never go away and you'll end up wasting a lot of time.