I've recently started going down on my girlfriend with generally good results. I enjoy doing it and she loves getting it! However after a not very long time, say 5-10 minutes, my tongue starts to get really tired. And many times she asks me to go faster and I just can't physically do it! I've spoken to friends about it and they all seem to be able to move their tongues a lot faster and for a lot longer than me.

I was wondering, is there anything I can do like exercises of what not to increase me tongue's stamina and help he get better at going down on my girl?

Any advice would be appreciated.