Hi Wordgarden,
I'm a word person too, but I'll try to be brief. You've a lot on your plate and don't need extra verbiage.
Your partner's got cancer, right? How serious is it? What would he do if you got cancer right now?
My partner's love is very erractic too. It's his nature. It's unsettling and I don't like it, but it's there, overall. Is your partner's love there underneath all his angst?
You've been together quite awhile and he's being who he really is. Of course, that includes all the ugly parts. Can you endure them? Would you want him to endure your ugly parts? (I assuming you have some)
For the immediate future, assuming he does not need a great deal of care right now, can you get away for bits of time---weekends, or even a week's vacation? I'm spent a summer apart from my difficult partner and it helped me get perspective. If that's not possible, try just an evening away or a day. If at all possible, be with someone who feeds you in some way. Or if a walk helps, do more of that. Whatever helps you feel better, do it. Even if others say it's bad for you! (like eating chocolate or candy)
Unless your partner is really unwell, sex is probably very important to him. He's male and can't help that. It's a vital link between people who love each other---even when they are not being loving. Try to have sex with him. It can help restore the bond that is fraying right now.
Good luck!