I keep going limp during penetration sex with out ejaculating. Is something wrong?

Here are facts that I think are important. 1. First, I have watched a lot of porn and self masturbated a lot for the last two or three years. I had a a shitty marriage where I know I was losing all attraction to her, so at the time, I assumed it was because I started hating her and enjoyed porn more. But the problem had persisted with other women. I had also spent a year in Afghanistan where self please was the only place I was getting it, so I got even further addicted (or maybe even dependent on it). 2. Oral keeps me hard usually. When I perform it,i get semi erect (love giving) but if I try and go to vaginal, I can stay hard for a while, enough to make the girl go a few times, so in that department, at least she is getting hers, but then I get soft when I get close to finishing. I mean, I love to boast stamina as much add I can but it gets frustrating when I can't finish, and I feel embarrassed cuz she starts thinking I'm not attracted to her. And the last time I had sex, last night, I stayed hard in missionary, but switched to doggie, and it stayed going limp almost immediately. I stayed hard in 69 but she was more about me giving her. I think I was getting close to finishing in her mouth, but she was all into hers last night. Still went back to vaginal though, and I couldn't stay hard, especially during doggie. And what's worse is it used to work fine when I was 20, doing doggie, but I've never actually finished in doggie. But didn't go limp either. Now i'm 27. Anyone else experience this? Is it bad I can only finish when getting head or jerk myself off? Well, yes its bad, certainly frustrating, but is it normal, and how should I fix it? I've already forced myself to avoid porn, though last night out was painful that I couldn't ejaculate, so I had to resort to it after she left. I'm trying to not masturbate when I think i'm in proximity of having sex within the next few days. I'm trying to all together, but that gets hard during a sexual dry spell. I don't have many different partners, just one new one every six months or so. ... I think I it all.