It happens with all the men as that is a normal thing. When you masturbate yourself, you know how to do it and when to increase the speed. How to hold your penis and how much pressure you require but your girlfriend would not know about these things. When I was in the college my girlfriend masturbated me in the cyber café and sometimes in the park too if we found a private place and I took longer to cum when she did it with me but I was enjoying as I did not want to cum premature.
I had to tell her tight the grip and increase the speed when I was about to cum and that were the best moments of my life. However, with times she learned what she needed to do to make me cum quickly. Most of the time she masturbated me when we were in the cyber café as that was the best private place for us and she kissed me and started giving me masturbated and within no time I used to cum.
Therefore, do not worry if it takes longer to cum as your girlfriend in days to come will become expert how to make you cum quickly and mark my words that you have to tell her to give you maximum time as you would love her hand on your pens for longer time so enjoy the moments you are having with her without analyzing too much.