Hey XX -- welcome to Scarleteen! We're happy to answer your questions but first I need a little more information from you. How do you know or how did you come to believe you have phimosis? Is this not being able to pull back your foresekin something you've always noticed, or is this new? In general, it's a little risky to self-diagnose - especially a condition like phimosis, which is sometimes caused by having an infection inside the foreskin. I highly recommend seeing a medical provider who can give you an exam and help figure out what your options are for treatment. Is this something you have access to?
To answer you questions --
Is it a bad thing?
Phimosis is not life-threatening, but it needs to be treated. So no, not a bad thing per se but it's important you figure out what's up, especially if you do need some kind of medical intervention. It's possible that you do not have the condition and this is just a physical difference, like how people's noses look different from one another. It sounds like investigating this further will ease some anxiety for you.
Will it be uncomfortable/hurt during sex?
It's my understanding that it may be uncomfortable for you to have sex. It appears to be a little individualized, as in some people don't experience it and some do. This would definitely be something to talk about with a medical provider.
Will it be a major turn-off during intercourse?
It's difficult to generalize the experience of being turned on or off, and it would be wrong to assume anything about how your girlfriend would feel about it. I think priority number one should be figuring out if you actually have phimosis and what the treatment plans are. You can share this health concern with her, or you can simply say you're not ready at the moment. There are plenty of ways you can have sex and be sexually intimate with her that don't use your penis,
you can read about that here.
Should you go through with a procedure to remove the foreskin?
This is something you should definitely speak to a doctor about, as procedures like circumcision can alleviate phimosis and are commonly prescribed as a treatment. It may come down to your personal preference as well.