I've Had Sex Only One Time out of my Whole Life,And I Have a whole new situation. HELP!?

[Alex]

Alright, so here's the problem. I've had sex only one time in my life, and I didn't have a full erection. So let's just say I Had intercourse with a "Softy". And That very first time, I Wasn't too experienced because I Lost it to a girl that wasn't a virgin. Worse yet, I Didn't exactly "cum". So now, I Have a new girlfriend, And we're Both ready to take it up a notch. But now the problem is , whenever we try to have sex , I Would sometimes lose my erection. I Lost my erection when it comes to using condoms. And to make matters even worse, when I Actually did have an erection, My Penis just wouldn't go into her Vagina. She's a virgin, so I'm gonna guess she's too tight. But at the same Time, I Need to know how to keep myself from losing my erection too quick. I Also need to know why My Penis won't just slide into her vagina with ease. Guy's and Girl's, PLEASE HELP ME!