I'll throw my non-professional $.02 right in with the Love Goddess. She gave lots of good advice on many related issues. I'll give some from a guy's point of view, perhaps not that different than your own.
If everything physically works fine during masturbation, then it is in your head. That doesn't make it any less frustrating. I feel your pain, as I've been there. Lying naked next to an attractive woman, wondering what the hell is wrong with me. Getting frustrated at yourself because you know that she isn't having a good time either. It isn't fun. But the more you think about it, the worse it gets. Trust me, I've been through that cyclical thought pattern.
What has been helping me over the hump, so to speak, is patience. As she mentioned, book a longer session. If that means less often (for financial or scheduling reasons), so be it. Tell the lady the deal, so she can adjust accordingly. Then, do your best not to have an agenda. Don't make a mental list of all the things you are going to do with/to her. Try to go with the flow and just do what feels good.
For example, I've found that one thing I really enjoy is kissing. In some ways, that works better on my brain, and therefore the rest of me, than many other things. If I walk in the door and we spend some time kissing, that helps. If she starts running her hands over me while we are kissing, so much the better. There are times where this is much more effective than a sloppy, slurpy BJ. That isn't to say I don't enjoy that too, or wouldn't go for that a bit later once I'm ready.
I'm not saying kissing is your thing, or the ultimate answer. That is just one thing that gets me going. Your answer will likely be different. It could be her talking dirty to you. It may be you licking her breasts. Whatever. Neither the LG or I know what turns you on. That is for you to find out, and it won't be the same every time, or with every woman. But slowing down and worrying less about the erection and coming, and more on simply what feels good will be a more productive approach. Don't worry if you end up spending some time (and money) learning to do this. The effort will pay you back over a long and enjoyable time. So don't rush along with crazy expectations. Build patience and skills.
If you mentally get to the point where you would be perfectly content to spend the couple hours with a lady simply touching, exploring, licking and rubbing your body parts over hers WITHOUT intercourse, then your relaxed brain will get the hell out of the way and let your body do what it is pre-programmed to do. It is a zen thing, or like the children's retort of "when you least expect it, expect it". When you aren't worried about getting the solid erection, you will.
I haven't mastered these things myself. But I'm a couple steps farther down the road and I see the light. So give it time and, paradoxically, don't try too hard. And do give a read to the book the LG recommended. After she mentioned it in a different post, I picked it up used and have been working my way through it, with exercises. Good luck and enjoy the journey.
While I'm here, let me throw out two mentions of thanks. First, the Love Goddess, for the good advice she didn't even know she gave me, because I didn't ask the questions, but read the answers. Second, to any of the ladies I've seen or will see that happen to read this. Thanks for your patience while I've worked on mine.
Ren'man