Hi cmf,
From what you have told me and being on the outside looking in, I would guess the loneliness you feel is for the ideal you want, to have someone to live with and share your life with and a security of sorts.
At times, your gives you some of what you need. As you describe above the way he extends himself to try to help, to show he cares about your dream would, I expect, make you feel cared for. It gives a feeling of security because he is being there for you.
The problem is that the next time he might not be. The next time might be a lunch that he runs over an hour late for or a promise to babysit but something comes up that is more important to him and that sort of thing. When these things happen the feeling is not one of being cared for, it then makes you question your needs.
To me, it seems this arrangement fulfills some of the needs you have but not all of them. I would guess some of the loneliness stems from the gaps between this arrangement and a fully committed, secure, relationship. Having someone that can be relied on all the time not just when it suits him.
To me the question would be "is it enough?" It sounds to me that it is not enough. If it were me, I would be looking at dating again, online or otherwise to fill the gap. At the very least if you do meet someone else, the way the ex reacts to this may give you the answer you need.
I am no expert, just my thoughts. Kind ones xx
