[22/m] I just realized that my girlfriend [22/f] of 4 years isn't the person I want to spend the rest of my life with, but I am to her. What do I do?

Hey, long time haunt, first time poster. Just a few details: we currently live together and have for over two years, we both enjoy it and get along great, she really is one of my best friends but we've been fighting more and more lately, things are becoming strained. I still love her and can't imagine waking up without her, which makes this realization all the more shitty. This whole thing is pretty liberating and soul crushing at the same time. We're suppose to have a big relationship talk tomorrow and I fear things are going to go south pretty quickly. I'm not sure what to say or if I should say anything at all, but I really don't want to lead her on into thinking this is going to become marriage/permanent. Any advice my fellow redditors?