I have a hard time cumming when I have a hard on

I am uncircumsized and have my own technique that i use to get myself off that is different that the average male's. I just go for the g-spot on my head and just rub it and eventually it happens, and i usually cum, but i can't do that when it gets really hard(full erection can see head) when i usually masterbate i just get a partial hard on, or some times it is even flaccid when i cum. Here's the problem now, when i get a full hard on it feels wierd because as an uncircumsized male I'm not used to having the air touch it, and when i have sex it's painful, like my penis get's red it feels like its taking a beating. I have no STD's so I'm just wondering why is it painful for me to have sex and what can i do to possibly get rid of the wierd feeling i have when my head is free when I'm hard, and finally what can i do to start getting into the habit of cuming when i have a hard on?