Erections Only Happen on it's Own

I’m 25 right now, and for a few months now, I have not been able to “get it up” on my own. My girlfriend can’t even get my penis up by rubbing it or giving me oral sex. It’ll just go limp.

The only way I’m able to get it up is really just by laying next to her while we are both naked and kissing. It’s so strange. She is my first girlfriend and this has been an issue for the last 3 months (since I got her). Before her, when I just masturbated, I was able to get it up no problem. Now, I can’t even get it fully erect while jerking off.