Replying to My dick wont stay hard

Ok so I got a fucked up problem thats making my life miserable. Whenever I try to have sex I either cant get a boner, it dies when i try putting the condom on, or it just doesnt get hard enough to penetrate.

This has just been happening with this new girl ive been fucking around with. I can always stay hard when i get head for some reason though. I know its not the girl cause she is very attractive and i like her as well. Ive never had this problem before but the last time i had sex was probably 6 months ago. I dont know why the fuck this is happening considering im 18 years old, in good shape, and dont have any health problems.

Even though i cant get hard, if she dry humps me for long enough i can still cum. if that makes any sense...
ive been thinking about what some causes can be so yesterday i quit smoking weed and cigs and im gonna wait 2 weeks and see if it makes a difference. Also i hardly jerk off, maybe once every 2 days. I do drink alot but i dont think thats the problem considering the same thing happens when im sober.

I feel like it cant be a psychological problem since im generally a happy confident person. Im not emotionally attached to any ex girlfriends either. i just graduated highschool so im at a pretty good point in my life and have no worries besides my stupid fucking cock that wont get or stay hard..

Someone please give me some tips on how to resolve this. I really like this girl and i just wanna bone the shit out of her but thats not possible with a flaccid cock. its really scaring me considering im 18 and i still have my whole life ahead of me.

sorry for any misspellings, i have worse things to worry about.