We're all there hon, we've all been there or we are now... .there are no answers, I honestly don't think our exes/SO's know themselves why they do what they do. I feel so sorry for my ex, not as sorry as I feel for myself obviously, but he has an attractive, understanding and loving woman (that's me by the way

) he could open up to and share his life with... .that he did share his life with... .and yet he has picked me up and discarded me on several occasions.
I totally resonate with your post, I am still at a loss as to how my ex could waltz back into my life after 6 months of ST, spend 3 intense and passionate nights with me, kiss me goodbye before going back to his home country for a holiday and then return behaving almost as if none of that happened. You can see from my other posts that there was a significant incident in which a so called friend of mine actually attacked him... .but it is me who's suffered the consequences. I'm right back to square one as he now walks past me in the street as if I don't exist... .the same man who some weeks ago was holding me in his arms and was desperate to show me that he hadn't had another GF since me.
He also 'stood me up' last year, if you want to put it like that, just as you experienced. On our first recycle, when we had just got back together for a couple of weeks, for the first time he'd started talking (by text) about his 'injured brain' and how I could perhaps heal him, and that he needed me. When I tried to get him to expand on this in person, he shut down on me. The next day we arranged to meet in the evening, but from what I could work out, he deliberately went out drinking and effectively got back too late for us to do anything, choosing instead to go to sleep. I waited for 3 hours for us to go out and had made a huge effort to look good etc. He somehow tried to make this sound like it was my fault. The next day, he stopped talking to me again, completely. A week after standing in my flat and beaming from ear to ear telling me how happy he was to be with me. 3 weeks after standing me up, and after I'd given up hoping he was going to talk to me again, he bumped into me on a night out. I wasn't going to talk to him, having taken the hint, but he pulled me to one side - to tell me that we didn't work because he knew how much I loved him and he would never feel as strongly.
7 months later he was telling me how much he loves me. Just days after telling me that he hates me.
I only tell you all this to tell you that you're not alone, you are not losing your mind, these things really have happened, but we won't get any answers as to 'how could you do this to me?' The answer lies in what you yourself have written... .they have a condition. A very very sad one, for all concerned.
The more times he's done this to me, the less it shocks me, I suppose. The solace I find is in knowing that this is NOT malicious, it's all a defence mechanism. They're not thinking about us and the hurt being inflicted, just about mitigating their own shame, self hatred etc. I find it easier than ever now, on my whatever time it is of being ignored, to know that this really is about him and not me. I also know he will talk to me again at some point. Until then, I crack on with my life.
Just like you, I miss my ex as well, terribly. I can't seem to find anyone even remotely suitable to take his place, so I'm not looking. I really am just biding my time and concentrating on trying to do things for me. My ex lives just minutes from me, but stepping to one side and trying to look at this objectively, see this condition for what it really is, and knowing that they're not actively trying to inflict pain, really has helped me to get stronger and feel better. It hasn't stopped me loving or missing him, that's something different.
Everything will come in time.