"Oh no, what if X happens, what'll I do? I might lose Y!"

I find that asking and answering a couple different question helps.

"What'll I do if I lose Y?" Not gnashing your teeth over losing Y, but actually answering the question. "If I lose Y I will ..."
"What's my best strategy here?"

One claim in some depression book - catastrophizing is at bottom a worry that you won't be able to handle the loss. It's a boogeyman in the closet, and you're too scared to open the closet and check, so live in fear, night after night, that the boogeyman will jump out and get you.

Open the closet and look. The boogeyman is there or he is not. Do you think he won't come out and kill you just because you refused to look to see if he was there? Buck up and look. What you'll likely find is that the boogeyman really isn't so fearsome, and your fear of the boogeyman is more crippling than anything he might do to you.

Either you'll spend from here to eternity with this fantastic girl, or you won't. Likely not. Someday you'll lose her. You'll weep and moan and move on to some other girl in a few weeks. Or maybe you'll dump her. You'll look back on your time together fondly, perhaps guiltily, when you're with someone else. Or perhaps 5 years from now you'll be struggling to remember her name.

Closet opened. Boogeyman faced. Situation handled.

Now, you look at your options in a game theoretic sense.

She's on the verge of dumping you, or not. You can fret about, or blithely go on as if nothing is wrong. Fretting makes either situation worse. Dominated strategy. Do not do.

You can handle it if she dumps you. Trying to read the tea leaves for the impending dump only makes you look and feel insecure. Do not. She'll eventually let you know if she wants to dump you. Until then, you're seeing a fantastic girl. Make hay while the sun is shining.

I'm a worrier. I'm trying to get over it. Part of it seems like fear is a response to uncertainty. Remove the uncertainty. Face the issue, pick a strategy, and move on.