How to finish from head

I have never been able to finish from oral sex and I was wondering if anyone had insights to help me. I can always get and stay hard, but everything starts to feel numb and then I kind of want it to just be over and I start to pity my gf putting in effort for nothing. I feel like i clench a little I don’t know how to let that go. I feel like I should be able to cum easily from head but I feel weird and bad that I can’t. I start to get upset at myself and then sometimes her (which is not fair) so now I avoid head or only use it as foreplay if I feel like it sometimes. She really wants me to finish in her mouth but I don’t know what to do and also how to not feel bad about her going down on me cuz I know it’s something she wants to do and it just kills the mood