I'm really sorry this has happened to you, anyone who has used OLD has been on the receiving end of this kind of treatment at some point, you think that it's going well then poof it's over, sometimes you're told sometimes you're not. I was in a seven year relationship that ended with no warning. I came home from work to find that he'd cleared his stuff and left the key, I never heard from him again, all those years gone, without an explanation. Tbh I did hear from him again, a text a few months later, trying to reel me back in. I replied thanking him and telling him I was happy and not interested in remaining in contact, wasn't true and hurt like hell but I realised I didn't need answers, if he could do that he wasn't worth my time, if I can cope after that length of time you can. Sometimes there aren't any answers you have to just deal with it and accept people change their mind and will do what's right for them.
If I'm honest I waa also in a similar situation but the other way round. I met someone a couple of years back via Old, we got on, I liked him but looking back I wasn't ready for a relationship, I just didn't know that at the time. I spent a lot of time with him, it was intense, I got so wrapped up in it I convinced myself I loved him and told him so. The thing is as soon as I had some time and space I realised it was in my head, I didn't want to be with him at all. I was hurting from a previous break up and found myself in a situation that I really didn't want to be in.
I did the retreat, being offish, hoping he'd start to feel the same but it didn't work, I did try not responding for a few days, I felt bad but I was so scared of hurting him I just hoped he would begin to feel how I was and would see it for what it was, a short lived infatuation. Eventually I met with him and told him it was over, I didn't want to tell him I had realised I never felt anything, that it was the situation I was in that was the problem, I didn't want him knowing I had come to the realisation that actually he hadn't really meant anything to me, it was just lust. In my misguided mind I was trying to prevent pain for him. I did end it though by being half honest and saying it was the wrong time for me, that I needed space to be single. I was clear about that and I that I had no intention to contact him again. He responded with all the things the op did, he would give me space, I was just scared because of how I felt about him, he would give me space because he knew I loved him really. He would text and call, asking for answers, me trying to explain didn't help. Looking back now if I could I think I would definitely consider just cutting contact, trying to tell him didn't make him feel any better, it didn't change anything for me or him and tbh even if I'd had any doubt those two days where he was calling and messaging me just made me loose all respect for him anyway, we had only known eachother five minutes and I just felt harassed and as though he had no self respect, I just wanted him/the situation to disappear. Not nice I know, and completely my fault for allowing it to happen when I wasn't ready but looking back I still find it hard to believe how he acted, I would never have done that over someone I hardly knew. I'm not a nasty person, I don't have mh issues, I'm not an insensitive person but I have made mistakes and found myself, on this occasion, getting caught up in the moment then realising there was no substance there and it was all a bit silly.