“if you did not get so alarmed by having those thoughts, if you took their power away, that could help very much. When you have those doubts, does it make you think you have to do something as a result of the thoughts? And the doing something scares you?”

You really hit the nail on the head.
I’ll describe how it used to be with my ex and how it is now in my current relationship.

With my ex the thoughts were usually centered around the worry that I didn’t really love him, that the relationship wasn’t meant to be because he was somehow wrong for me. Because I never felt in love with him in the beginning, and he was the one who insisted more on us dating whereas I wasn’t so sure, I later felt that this meant the relationship was somehow not “valid” or “real”. That I wasn’t really attracted to him and was only with him because he pushed me into it and so on. There might have been some truth in that, but I have stayed with him for a few years and endured these thoughts all the time and was constantly anxious. Whenever I would think about that (which was most of the time), I would tell myself that these thoughts meant I had to break up with him. So the worry and rumination were about – should I break up with him or not. That made me very anxious.
At some point it became too much and I ended it and felt mostly relief.

Now with my husband, the thoughts are more centered around a specific “flaw” that he has, or at least it’s a flaw in my eyes. I keep thinking about it and getting very anxious about it, comparing him to other people, even to people in the street I don’t even know, and keep convincing myself that he’s somehow less valid as a partner for me than almost anyone else. I know how crazy this sounds, especially since I’m not even talking about a physical flaw that can be seen right away on strangers.

When I get really bothered by these thoughts, I am no longer tormented so much by the fear of “having” to leave him because I have already decided I won’t and chose to marry him, so it is something I know I don’t want to do (break up).
I am however petrified by the idea that these worries are the truth – that I cannot be truly happy with someone like him. And so my thoughts this time around are less centered on an action I feel I need to take, they have taken the form of determining whether or not I could still be happy with him. Although in these moments of extreme anxiety it seems to me there is no question about it – that I obviously cannot be truly happy and fulfilled with him and would therefore be doomed to leading an empty, unsatisfying life where I only pretend to love him and actually be dead inside and lying. It is however very very disturbing as I then feel I am stuck in this situation and there is no way out. The “action” in this case presents itself more as the necessity to “look reality in its face”, so whenever I’m happy with him or laugh from something he said and so on, if I’m entering this state of mind at the time the thoughts would surface and say something like “you think you’re enjoying his company but you really are just compromising and settling, it could be so much better with someone else, stop lying to yourself” and so on. And it’s making me very anxious. It really does sound like some creepy mental illness… which is less scary to me than the idea that these thoughts are right and true.

I’ve had similar feelings and worries in every committed relationship I’ve had or every budding relationship where I knew the guy was interested in me. And the “action” thought was always about breaking up. Sometimes I didn’t even know what was bothering me in the other person or the relationship, I only felt this anxiety and strong feeling that something was wrong and I needed to get away from the person and not see them anymore. I did not get this so much, or not at all, in relationships where I didn’t feel secure in the other person’s interest in me. But these usually ended quite quickly anyway.

Sorry, I wrote quite a lot…
Thank you for listening.