When you are with someone and all you can think of is them and no-one else, you've met her!
I disagree and here's why. I felt this same "obsessive thinking" or "emotional high" with most of my girlfriends. It was mostly due to New Relationship Energy, or the Honeymoon Phase. But not many of them were compatible enough to be a life mate. So, to tell if this person would be my life mate, I think "What if all the NRE was gone, I would still love them. But would I still LIKE being around the person?" If the answer is Yes, then they might be my life mate.
Being familiar with someone, or being afraid to find a new mate, is not love! And it does not mean they will be compatible enough to be a life mate.
Next, Zertaag, you are still young and it is normal to want variety, especially if you have a higher sex drive. But there are consequences to your actions. Thoughts about another woman do not mean you do not love your current gf. You simply might be curious about another type of woman, or another "type" of cuteness. I might have a slim blonde girlfriend who is pretty hot, but I would still be curious about a brunette.
You need to determine if exploring this is worth breaking up with your current gf. Or maybe you can work out an open relationship. It's not for everyone, but some couples make it work. For some, exploration and adventure and variety are a major thing they really need. You should determine if this other woman is a passing curiousity, or a major need in your life for some adventure. I mention this because if you don't get your major needs met, you will be unhappy.
I have a long time interest in psychology, specializing in relationship dynamics for 30 years.
(Please note, we give the best advice we can based on the information given in a post. For better advice, please include the age of all romantic partners.)