My relationship has an expiration date - how do I enjoy the rest of it?

The TL;DR of this is that we are together for the next month, at which time things will end. I would like to enjoy the rest of our time together rather than spending our last month together in mourning. How do I keep myself centered?

As for the long form, this is mostly for my catharsis - not sure that it will help you with the advice to read through it. But while what I really want is the advice, I also want to get this off of my chest.

I'm 35, and have been dating my partner for most of the last three years - the past 17 months have been a "real" relationship, whereas before that, we were on and off casually.

We both went into this knowing that the relationship couldn't last as it was. He currently wants kids, I do not. We both know that's not compatible. We both have our own things going on and need to figure them out. The relationship as it has been makes a ton of sense and neither of us regret pursuing it. We have both supported one another equally, helped one another grow and all around been great partners to one another.

He is not happy. In life, I mean, not just with this relationship. He doesn't feel as though his job is fulfilling. He admires that mine is. His main reason for wanting kids is because he wants to feel like he has made a lasting impact/goodness on the world, and he is not 100% sure that he would want kids if there were something else in his life that made him feel that way. I have held space for him to find that thing in the time that we have been together, but he has not. I do think that there is potential for that to happen and for us to have a future, but I did not go into this trying to change him or change that, either. That said, it has been very hard not to try and "fix things" or push, and now I do have some regrets that maybe I should have done more.

I am a bit of an overplanner, and my love language is acts of service for others. I know that because of that, he feels like our relationship is unequal, with me putting in significantly more effort. I don't feel that is true, but it is true that some things have changed for me as a result of this relationship. I used to pursue my hobbies and build my social life around that, but now I build my social life around spending time with him at the expense of some of my hobbies (which we do not share). I work about 80-100 hour weeks, so I simply don't have the time for both, and I prefer to spend time with him. I do miss my hobbies, but in a couple of years, my work hours will settle down and I will have time for both. The reality is that I enjoy time relaxing with him more than I enjoy playing a board game. A lot of my hobbies were there to fill the emptiness that I feel when I am left to my own devices.

I have been in several relationships before, and he is the first person I have ever dated who made me feel as though he was a true "partner" and that everything wasn't on me. He undervalues himself significantly. He tells me frequently that I deserve someone who can love me, who wants to plan as much as I want to plan... but I don't think that reality comes down to what someone "deserves."

I love him, and he does not love me, which I have been okay with: my love has grown over time, and regardless of what he feels, he makes me feel loved with his actions. He has never promised anything or lied to me in any way about what he can offer.

I will be out of the country for a few months, and I felt like this would be a great window for him to have some time to figure things out for himself. He's here with me for the first little while - it could have been two months, but he's going to cut it down to one and head back early. I understand why. It all makes perfect sense.

He thinks that I deserve more, but I have my own shit to figure out over the next couple of years, and I do not want 'more.' I do not plan to pursue other relationships for the next couple of years (I will not have time) and on top of specifically being sad to lose him, I am also sad that I will be lonely again. I have been lonely my whole life, and that went away with him. I eat better, I exercise, I handle my stress better and I manage my ADHD and depressive tendencies well. He feels as though I have helped him significantly to be less unhappy, less lonely, a better person with his family and friends, and that I have supported him in pursuing his side hustle (which brings him joy in a way that his work does not). He has been there for me through one of the toughest times in my career, and I am sad that I can't be there for him while he is figuring out his shit as well.

I am just sad in general, I think. I had hopes that he would agree that he needed time to work on himself, but feel as though he wanted to see where things went when I was back. He doesn't want that. And as much as I understand that, it is still very hard.

I know that in the unlikely event that we are meant to be together, things will work out that way. And I don't want to stop him from finding happiness. But I'm having a hard time putting off the grieving/mourning period that is normal after the loss of any relationship. And I knew it would be coming, and it's a price that I am very happy to pay for the beautiful time that we've been able to spend together, but I'd like it to get the hell out of my way and let me enjoy the next month. I know this is a fairly atypical situation, but if you have any thoughts on strategies, I'd love to hear them.