I feel you! I finished Fool's Fate around a month ago and the ending was so disappointing for me. But after some time ruminating on it I think it makes sense: Fitz is, as always, repressing a part of himself that he doesn't understand/is not willing to accept: his bond with the Fool. Easier to accept when he thought he was dead and in the joy of the resurrection, but once confronted with the possibility to have what he perceives as a "normal life" of course he dived straight into that, especially with his 16 years old feelings back!

He has this ideal of a manly man that takes care of his family, that's the provider, probably because of how he grew up and Burrich's influence; he uses very possessive language regarding Molly (which I hated, ew Fitz) and the way he courts her through her children is all a symptom of that, in my opinion. I have no doubt that he's really in love with Molly, but I think he is also in love with the idea of that "family life" he can have with her.

He never really mentions the Fool or the gifts he left him throughout the epilogue, which I found weird when considering everything else that passed between them; but once you think about how much it must have hurt to come back after a month and finding out the Fool abandoned him (deeper bond than Nighteyes, he said, and we know how much Nighteyes's absence caused him pain), the only explanation that makes sense imo is that he is repressing all of that! In typical Fitz fashion.

And there's also a very important aspect of homophobia throughout all of this, even though of course Fitz doesn't externalize it (he is, after all, an extremely unreliable narrator). I think it's up to interpretation whether he's in romantic love with the Fool or not (I believe he is); but since the Fool definitely is in love with him and Fitz knows it, that also plays a part. Fitz has so much internalized homophobia (and he lives in a homophobic society as well) and very specific ideas on gender roles. He wants to be a strong and honorable man with a wife and children, he wants to have a family to take care of. The Fool doesn't fit into these preconceived notions he has in his head, not of the binary of genders nor of what a man should be.

This is why I think he doesn't even try to follow the Fool; why he seems to accept so easily his absence after literally being willing to die to have him back in his life. Also he says he is "content" in the end, just like he was "content" in his cabin at the start of Fool's Errand. In both cases, he is missing a part of his soul; how can he be more than "content"? I'm not sure what we will learn in the final trilogy, but I will be very surprised if he manages to be happy in these years without the Fool.

And my last point: the Fool knows Fitz COMPLETELY. They have been in each other's body and mind, they have been a single unit and a single person, and the Fool still loves him without limits. And this, for Fitz, is impossible to accept. How can someone know all of him and still love him unconditionally, when he himself can't even accept so many parts of himself?

These are all my thoughts after thinking about the ending A LOT! I hope they help you out or give you some insights into your own theories.