It is scary!! I hated hearing this before I transitioned but give it time! I remember how anxious I was about coming out and I spent a long time questioning but you will know what you are in time. For now, have fun with it. Gender is an art form and if something makes you feel good, do it. I’m transmasculine and started my journey with experimenting with clothes and masc makeup, just do what feels comfortable and you can figure the rest out later
I felt this way too for a long time. It’s hard to go from “I am cis” to “I am trans 100%”, there’s a lot of societal pressure and expectations you have to fight through in your mind to get there. Just keep experimenting with it, try out new things if they feel good to you. Maybe ask your gf to try using she/her pronouns with you and see how you feel about it.