I think I might have fallen in love with another girl and it's starting to make me sad and depressed
Thats not good! Why do you think you've fallen for another girl?
Well I'm starting to miss her and I can't stop thinking about her, for instance when I'm hanging out with my girlfriend.
It's reasonable that you feel sad and depressed because you're falling for a girl who isn't your girlfriend. These feelings of frustration are normal when you're experiencing a confusing situation.
Oh god, does she love me too? Shit.
You should take time away for yourself and figure out what you really want. It's unfair to your current girlfriend to have feelings for another while you're with her. You have to choose who you want to be with and end it with the other girl so that its fair for everyone to have a new start.