Am I being stupid and jealous?

So I've been dating a girl for about 6 months now, we're taking it slow since it's the first same-sex relationship for both of us. I could be completely stupid, or paranoid, or just plain jealous (please tell me if I am), but from my perspective she seems to want to spend time with her best friend a lot more than with me. He is transgender, and is a nice guy, but he's always over at her house, and frequently sleeps over. I guess it's probably just a normal, close friendship, but my girlfriend doesn't seem to ever ask me to stay/hang out with her as often/for as long as she does with him. I just wish she treated us equally I guess...

Any advice on how to get over myself/talk to my girlfriend about this?

How old is she? Was she in hetero relationships before? Were you? Why don't you ask her to hang out more? Invite her on dates or invite her over. Is it about this friend or lack of time together?

it's the first same-sex relationship for both of us.

she seems to want to spend time with her best friend a lot more than with me. He is transgender

my girlfriend doesn't seem to ever ask me to stay/hang out with her as often/for as long as she does with him.

There needs to be a healthy balance of time alone, time with a partner, time with friends/family/career and hobbies. If you're feeling your companionship is lacking in time spent together, then maybe you're in the wrong relationship. I don't even think I'd have a discussion with my partner about this, because as the saying goes: Your feet take you to where your heart is.

Why beg for time with someone who wants to spend more time with someone else?

Maybe her friend is not very comfortable around you (in matter of discussing personal stuff and such). What I would do is talk to her and tell her you would like to know him better bc if someone is very important to her it is healthy to have a good relationship with him too. Tell her it would be fun to hang out all together. Maybe he will become your friend too!!