I have known people for years before I realized their name was actually a nickname. Some people just don't ever use it. If it's an unusual name too, and very different from the nick, I see how this can happen. Also, credit to you, OP for feeling bad for forgetting, and making an effort to find out. You're all right.

Yeah, when the FML from the flip-side sounds more FML than YDI, then... well, YDI.

better then calling her the wrong name

how long have you been dating?! that's ridiculous.

That's a side hoe if I've ever seen one! Just to be safe, call her Felicia ;)

Not to be the devil's advocate, but to try and understand a way this could happen for op: maybe the girlfriend's name is super complicated (and nobody uses her actual name)? I had a Polish friend whose name I could barely understand and pronounce. His nickname was a bit easier, but amounted to a full name imo. So even if she's his gf, might be that they're not from the same cultural background, and he has a hard time even understanding / pronouncing her full name.

It happens to me sometimes... I just don't forget it 100%. YDI, OP.

Ask her how she writes her name in cursive?

I can't call my sister by her actual name now without it sounding odd to me.

maybe try to be a good boyfriend and remember her ******* name.