Weak Erections Help

So to make a story short. Im 22 years old. Im a little overwight but work out a lot. All the girls im with say im not fat just a big bone guy but anyways im just not happy with the way I look nor im happy with my penis.
So I have been dating this beautyfull girl for about a month now, she is the prettiest girl and sweetest I have been with so far. So last night I go over to her place and we where watching tv on the couch. Now so you guys have an idea of how it was going, I have been in this situation before with other girls since this summer.
So we are watching the movie and she leans on me and puts her head on my shoulder, I started to caress her and gave her a head rub which she loves and I started to feel aroused (you know it starts to grow a little) so im happy at this point because I thought it would happen last night. So she starts kissing me and we start getting more and more passionate for eachother. She sits on top of me and I get up and carry her to the bedroom. At this point other times I will have a boner already but since the summer nope. So we laydown on her bed and we get undress and all and boom there it is, I completly lost my erection, well never had one. So I start getting frustated but dont let that bother me I kept on going doing my thing. After about 2 hours I made her come 2 or 3 times with my hands, mouth, knees. But no erection for me. She wasnt unhappy but I was so unhappy at myself. I have been wanting to be with her for the longest time and now that I had one chance and probably the only chance, I blew it.

This have happen to me with 4 girls already after this summer and im starting to get annoyed. Before I will get good erections on my own but now not even that. When I masturbate I have noticed that I get only a full complete and nice erection when im close to ejaculating.

By the reaserch I have done I can conclude that my problems are either psychological ( since I have put myself down for my size 6.75 inches) and im affraid girls will judge me.
Or physical that maybe I over masturbated at some point. Or my testosterone level is low.

My friend who is in good shape and always goes to the gym has had this same problem and he tells me its stress but how the hell do I deal with it.

I know that I may have lost this girl after last night and thats ok I wont blame her but its me who im affecting. I feel that im not a man at some times for this issue.

Can anyone please help me on this im really starting to get depressed about it.