Seemingly Weird Problem, Help Anyone?

I'm 17, male, all around healthy. I've got an active sexlife with my long time girlfriend, but i've had a few issues. Sometimes I can't seem to get erect, at least not right off the bat. Always during foreplay i'm fine, but once my clothes start coming off, i've been going soft as of late. Sometimes after a bit things will firm up and be fine, but for a long time I didn't have this problem at all. I'm curious if anyone has experienced similar, and what may be the cause of this.


My best guess is something emotional/psychological, since it certainly appears everything is functional physically.


Anything comments at all are appreciated.