You really are in a tough spot, but rest assured, your feelings are completely normal. Here's the deal: According to statistics, there are actually 10,000 people in the world who would be perfectly compatible with you. I'm sure this other woman is one of those 10, 000 people! So the real issue here is making a choice and sticking with it.
No matter how much you like the woman you're dating, you will always wonder what it would be like to be with someone else. A man's natural craving for sexual variety always leaves him wondering what it would be like with other women. If you act upon the wondering voice, you will be a relationship rolling stone, someone who never establishes roots.
Ultimately, you need to decide whether you want to be a rolling stone or a person who is settled in a relationship? To settle on one person, you need to feel that the relationship you have with that person meets your needs on most levels. If it does, then why would you go looking for another relationship?
You may also be thinking that you have the potential to have an even better relationship with this other woman. Ultimately you have decide what is good enough for you.
We humans are voracious and insatiable and always wanting more. If you act on your hunger for more and better, you may get more or you may end up sorry and disappointed if you give up your current girlfriend only to find out that this other woman isn't good for you.
You have a tough decision to make. I hope that I have helped to clarify how you can go about making the decision.