Can't seem to masturbate at all?

I know this might sound weird, but I'm an 18 year old boy who can't masturbate. I have no religious views that make me feel against it, I figure everyone else does it, why not me, and so on. Sure I know it can be bad doing it to much, but what about not at all?

I have only had sex three times, and that was well over 2 years ago. Unfortunately no matter how hard I try, I have not been given another chance yet, albeit I haven't tried too hard. 

I had a girlfriend from 9th-11th in high school, during which we had sex twice, and once before that with another girl, all 3 times were normal per se. It was a good 20-30 minutes, stimulated  easily the whole time, and had no problem coming, or holding it back when it was needed once. 

I can't seem to get stimulated on my own, at all though. Any amount of porn doesn't help, whether it be pictures, books, the internet, words, videos, etc etc. I have tried it numerous times, almost kinda frustrated about it. I think it is perhaps psychological, maybe? The woman I have been off and on with for a couple years now, the reason previous relationship was ended, my first time, is often out of state, so I am left without that sexual opportunity, and she doesn't want to when she is home. It's not like we're married or anything, but we want to go to that step together, were best friends in that husband and wife manner, but that's getting too off topic.

We were webcamming one day and wanted to try masturbating together, that didn't work, at all. She was stimulated, she came, typical. I however was stimulated (which doesn't happen with porn of any kind, only with her or when I actually saw other women I knew, it's strange) but couldn't come. She continued to try and help, but nothing that time. We tried the same thing in person one day, we didn't do anything to each other, hoping I would be able to through pure masturbation, but I couldn't. After giving up, she gave me oral, and soon thereafter, of course I came.

Doesn't it seem weird not being able to be able to stimulate yourself alone to the point of coming, it doesn't make sense to me, and not being able to break this curse of hardly ever getting hard and even less of coming and whatnot, or whatever you wanna call, I feel I might need to seek medical attention of some kind.

Beyond the information there, only other useful information I can think of is that I have Muscular Dystrophy, Chacot-Marie-Tooth Type 1A, the most common form, not a terrible thing at all, I'm a normally functioning human being beyond the arched feet hindering me from playing sports, but that shouldn't have anything to do with not being able to make myself hard, right?

I need help D:

(Sorry for the long post)