what to when it feels true!??

What should I do when the thought feels true and my brain looks for evidence.
And I live that thought.

For example:

The thought and feeling pops up that I don’t love him enough. The romantic feelings aren’t enough.
That can happen when I talk to a friend or I have a hard moment at work. My brain is just like: we have a problem. The problem is how we feel about him. Its not enough.
And that reminder comes from some moments where I actually thought something I would describe like: ah he’s cute and a really good man but just not enough.

and when that happens especially when I’m with other people its hard to just see it as a thought.
I try to tell me its not the truth. but my brain is already making up scenarios and stories of how a girls life looks like having this thought meaning believing it.

The only thing that keeps me sane is that I know that last week I felt heartbroken, really really heartbroken of the thought that he wasn’t in love with me anymore and that someone else gets to love him.
And then every  time something good happened it wasn’t that good because I imagined that he wasn’t there for all that.

So I keep going forth and back between these stories.
But the first one is so strong because I practiced it so many times. And it gets to the point where I truly act on it.

I know you can say “just don’t believe it”
But just imagine something that you truly believe. Maybe is something or someone you love or a movie you really liked and then I tell you you have to believe that you don’t love it anymore or you don’t like it anymore.
It feels that hard.

I need some tools on how to not get overwhelmed by the thought.
Not to dive into that role/Act of believing the thought. I act like its a fact.