I can get erection very quickly but not long lasting, after penetration it stays hard about less than a minute. Once I get ejacuate , I cannot get ercetion in whole night, What can I do? sometimes I want to have sex more than once in night.
You may not be focus enough to get a great erection with your girlfriend. In my opinion, masturbation clogs the focus in the brain which explains why it is hard to focus on school studies after masturbating.
Why don't you try not masturbating for a couple weeks to see if that helps you get more sexual confidence, more sex libido, a clearer mind towards your girlfriend, and a better erection?