You're asking a group of strangers whether it's okay to leave your girlfriend. That tells me that you should place your priority on your startup.
It's like marriage - people say that you'll know when you know. If you're not sure if you know, then you don't know.
The same couple that "just knows" they are great for each other during great times will be unsure if circumstances outside of their control lead to more difficult times. Doubt is a natural and healthy response when conflicting responsibilities clash in life, and should not be interpreted as some sort of signal that there is no "magic" or that it isn't "destiny".
I'm talking about knowing you're _committed_ to another person. When you know you're _committed_ to another person you don't have to wonder if French lessons should go, or the startup should go, or the girl should go.
In theory I could know that I'm great for 314 different people, but that's a lot different than knowing that I'm committed to one.
This isn't fantasy and it's extremely representative of what goes into making a working relationship.