If it's okay, I do have some thoughts and response. First, it is very cool that you are able to think things over like this, even if you don't yet have answers for yourself. And I commend you for how compassionate you are, you want to be good to your friend.
This is all my own humble opinion and experience, so if it doesn't ring true for you, no worries at all. I'm 47, my son when he was your age wasn't sure about his sexual preference, thought he was probably gay or bi. I myself fully support everyone's rights to love whom they choose.
You are just 18, living with parents, you of course need to keep yourself safe and secure, that is a priority. Again, this just being my humble opinion, I believe it is fine that you are gay. I hope that you can find a way to treat yourself well, to respect yourself, to give yourself supportive positive messages.
One of the many mysteries of life that is wonderful is that we truly don't know the future. There are many, many possibilities out there for you. It may very well seem now that certain doors are closed to you, that some wishes can never be. But all the evidence isn't in.

The world has many gifts.
If your girlfriend can be supportive, perhaps you can tell her more about yourself. Ultimately, it is your decision. Please stay safe.
Sarah