First of all, OP, I'm so sorry. This is a horrifically difficult situation, and I think everyone on here's heart goes out to you. But I think dragon makes a good point that you should consider carefully before ending your relationship -- have you had any separate sessions with a good counsellor, or were they all joint ones?
I think the error some people on the thread are falling into, with the best of intentions, is thinking that following your dream of having a child is the only goal to be struggled and sacrificed for here, and that choosing your relationship over your wish to have a baby is a compromise you shouldn't make because it isn't being true to yourself. But a genuinely loving and happy relationship is also a huge boon, and I wouldn't in your shoes sacrifice it without careful thought, especially if you say you don't want to have a child alone via a donor.
In that case you are putting a huge amount of pressure on yourself and on any future relationships, to get to the stage of commitment etc where you could have a child, and without any guarantees.
You say you know you'll resent him if you don't 'at least try for a child, but flip that and you could find yourself resenting other men for not being him, or for not being open to a child, or for not wanting to discuss having a child early in a new relationship etc.
I'm not saying you shouldn't do it, only that you shouldn't fall into the trap of thinking that the only way of being true to yourself here is by leaving your relationship to try to have a child with someone else. At least see a counsellor and talk through your feelings about not wanting to go it alone, and how your reason for ending this relationship might impact on dating in future.
(Normally people would tell you to take a long time out of relationships, to recover and not to risk rebound relationships, but you are going to want to start trying to find someone soon, aren't you, for the same understandable reasons that you want to leave now? How will other men measure up to this man you really love? Will a willingness to have children make up for them not being him?)
I say this as someone who was entirely uninterested in having a child, and whose longterm partner changed his mind when I was 39 and suddenly desperately wanted one. I stayed, and had a child, more for him than for myself, if I'm frank. I adore our son, but I am also aware of the very different, equally good, life I gave up when I made that decision.
What I'm saying is that every decision means you close down some options.
Very best wishes, OP.