This is something you just have to push through.
What I suspect is happening is the mind just gets overrun with feelings that normally only happen once at a time. With one girl you’re like, “oh yeah I’d love to hit that” or with another one it’s like, “oh I could see myself raising a family with her.”
But with a crush, lots of different emotions compete for your attention and crowd out other kinds of thinking.
What I’ve found useful is to have little goals, and focus on those goals. For example, if you haven’t talked to her yet, make just saying hi the focus of this interaction. Once you do that, retreat. Next time, talking to her will be easy, but you now need to hold a conversation. So stick it out a little longer.
Do this enough times and you’ll eventually run out of things to accomplish. You’ll probably also be old and dead with lots of grandkids.