I'm having trouble keeping in love with the same woman. Sometimes my brain is on fire.
Why would you say that? Why do you feel your brain is on fire?
Because I always think of a woman who I can't have
Not every relationship works out, it is okay if it doesn't but you should try talking things out to see if you can resolve it. If you do you might be able to fix things.
I can't stand it. I have to have her no matter what.
If you want to be with someone else that is a sign your relationship is bad. The mature thing to do would be to talk to your partner.