Welcome to healthcare magic!
Your girlfriend is quite experienced and I think she is a bit demanding also.
These two factor are causing a performance anxiety
You are perfectly normal because you are having a normal erection in the start.
Please do not take pressure, you would soon learn to adjust and would be able to maintain good erection.
Remember the days when you learnt to ride a bicycle, you were fit still you fell so many times and now...
The same way you will learn to hold soon and would surprise you girlfriend with a bang!
You hardly need Viagra
in 50mg doses one hour prior to the act as required.
I hope this advise would be helpful for you.
Still if you have any queries or doubts, please write me back,
I would be happy to solve them.
Wish you a great health!