Easily attached

What you experience probably stems from some past relationship (doesn't have to be romantic) which you still unconsciously cling onto. Just gently remind yourself not to fantasize. Prioritise friendships over relationship, especially at your age (I'm assuming you're a teenager). You also have to remind yourself that if you're able to envision a future with every single woman, maybe you haven't found the right one yet. Rather than trying to have them fit into your fantasy try to wait and find someone who fits into it naturally. A mutual feeling.
P.S. Attachment is almost never healthy.

In 300 years time you'll be dead and forgotten. The universe is billions of years old, trillions of light years in size and will go on for billions of years. Your life is a speck of sand on the beach that is our solar system.
That's the mind boggling reality of the universe.
All these things you're stressing and worrying about don't matter in the grand scheme of things.
If your life turns out to be one long series of rejections, it doesn't really matter.
So you might as well adopt the mindset that if you're going to have setback after setback, you might as well relax, accept the setbacks with stoicism, and enjoy the process of going through setbacks.
You're alive now. You'll be dead in the next 100 years or so. Each day is a wonderful miraculous gift. Too wonderful to let stupid stuff be important enough to drag you down.
The only logical conclusion that you can come to, when you think about the nature of the universe and your position in it is that you should start taking yourself and your life a lot less seriously than you have done up till now. Especially all the stuff that you stress and worry about. Because even when the worst happens, it's no big deal in the grand scheme of things is it?
Being attracted to women is great. It's you observing the world around you and noticing the beauty in it.
What you should aim to be next time you're interacting with a woman is less serious than you have been previously. Because you might as well take enjoyment, pleasure, fun, laughter from the process of interacting with her - even if the interaction leads to yet another rejection.
IE you may well fail with her, but you're gonna have as much fun and enjoyment failing as you can.

Despite giving me a slight existential crisis, I like this advice. There’s this girl that I like but I’m not 100% sure if she’s into me, and I’m worried I’m going to get into one of these downward spirals again if it doesn’t work out. Do you reckon I should just ask her anyway? What do I have to lose in the grand scheme of things I guess

Oh I wish I was a teenager, it would probably make this less bad. I’m in my early twenties, but this is something I’ve struggled with for quite a few years. I can’t even think of a past friendship or relationship that it may stem from, but maybe that is the case.
I’ve often found that the right people will pop into your life when you’re not looking for them. Yet even then it’s never gone far enough to reach a relationship. I feel like I just keep finding incompatible people and that I’m somehow missing the right person for me.