Hey,
It's a good thing that you're able to talk to your girlfriend about this.
I think it's important that you try to figure out what it means to you, as a person, seperate from the relationship with your girlfriend as this is probably a very personal thing. Once you know what it means to you, you can try to figure what it would mean for your relationship.
If you figure out that this whole abdl thing is a real part of your life and something you don't want to ignore/surpress then you will need to figure out if your girlfriend would be accepting of that side of you.
It doesn't need to have have any real influence on your relationship with her if she's atleast accepting, you are still you, the person she loves, and if you want to wear a diaper every now and then it does not change who you are.
Yea, acceptance is key, goodluck with trying to figure this out =P