How do I improve my cock? (How easily I get erect, how long I stay erect, how quickly I can become erect again) I've had trouble getting hard and staying hard when I'm with women, even when it should be easy. 

After I cum, it can take forever, even ovenight sometimes, before I get erect again. 

I'm 22.  I should be able to easily get and keep an erection and get hard again shortly after cumming shouldn't I?

Sometimes it feels like a legitimate struggle to get and stay erect.  Even when masturbating. 

What can/should I do?