Why am i not able to hold an erection?

im 16 years old and i have been with my girl friend for a 4 months now
at first she didnt want to jump into things too fast but she changed and now we are trying to have sex.
well first to start i have used drugs(marijuana) in the past but rarely (only 5 or 6 times) and have used alcohol quite frequently (every other weekend) in the past (like 3 or 4 months ago) and rarely in the past months. i started to smoke cigars also but that was also 3-4 months ago and i didnt use it that much.  
so im with my girl friend and we have fooled around. she started giving oral sex at three months and i could hold an erection easily and we used a condom because of her concern of pregnancy.
recently she allowed me to "finger" her and the next night we tried having sex.
i had an erection but we fooled around and i go to put the condom on and i lost my erection. we tried so hard to get it back up but nothing worked. so then i gave her oral sex for the first time to break the tension.  i was so embarrassed.  but i told my self that it was because we had fooled around all day and i just prematurely ejaculated and then we were supposed to try the next night.
we tried and the same thing happened.  but this time i didnt let her touch my penis until it was time to have sex. so i performed oral sex on her again and i go to try and put the condom on again but again it is not hard.
i dont know what my problem is
i have been depressed earlier this year about my love life with girls cuz they broke my heart.

[youknow171]

also i used to masterbate alot before i met my girlfriend