It sounds like you have the awareness to notice when you are thinking about her. When that happens you have to do something different. You need a pattern interrupt of sorts.
When you notice yourself thinking about her, say to yourself "I'm not doing this anymore and I'm going to focus on the moment." Then observe your surroundings, feel what you're feeling in the moment, note what you're doing (oh, I'm driving right now).
Two seconds later you will be back on your fantasy of her. Repeat the above.
Do not make yourself wrong if you find yourself doing this all the time. We all have our patterns
. I know how it feels.
This is for later - it feels like the mental noise is covering up some emotion that's trying to come through in the moment. Excessive thinking is your way of not feeling it. That's why I said to check into you emotions during your pattern interrupt. There's something there not experienced and it's not necessarily 'bad'.