I get bedroom stage fright just as I'm about to have sex.

Hi, 

I'm a 17 year old boy. I have a girlfriend who I love loads but I'm worried about getting it up in the bedroom. i have had an opportunity to have sex in the past but i lost my erection just as i got the snake outta my trousers to put on the condom. I later got an erection again and got a ha****b. This was just a one night thing though. I have been with my current girlfriend for several months now and the same thing has just happened. We were already naked after a lot of foreplay ( I had already cummed from dry sex but I've cummed 3 times in a row before) then she said she was ready, as i went to get the condom, i lost my erection. we kissed for longer, but she couldn't do anything to get it up for me. when i focused on giving her the pleasure like fingering her and kissing her and licking her breasts i started to get it back up.  every time i noticed the erection coming back up again and thought about slipping the condom on and going for it i lost the erection. eventually after more kissing and rolling around in the sheets i had the erection up to full again and i went to i forgot about the condom then just went to push in. the tip went in then i remembered the condom, quickly grabbed it and put it on softening up a bit in the process, then i got the tip prodding in again.. we turned over so i was on top but the erection was still a bit soft so i had to squeeze at the base to harden up enough push the end in. i could maintain a near full erection for a bit, then we had to stop for a bit because she needed a break from the pain :/ .. then i lost it completely.. I know its not the condom which is the problem as i have tried masturbating with one on and it all works. also i do want to and am ready for sex especially with this girl, the thought of it can make me hard.. 

Please help i am quite worried about it.. much appreciated