[NSFW] How to convince my girlfriend for oral sex?)?

Me, and my girl friend share a healthy relationship for a quite long time.
we even share a healthy physical relationship too. Offlate I’ve notice that
she is not at all interested to start a thing first.Everything seems
to be depending on me.She enjoy very much while I perform oral sex on her.
but in return she seems to be pretty negetive. I know she loves me a lot but
this thing actually killing me and left over a depression. Any suggestion how can I
make her understand about my needs!