Not sure about the others, but Adventure Time ended up working out in my view.
A lot of AT was Finn's growth as he matured with time, Flame Princess ended up being a lesson in maturity for him. I actually like how they went back to it in later seasons. Finn apologizes to her in Season 5, but he is still clearly oblivious to the full damage and seems to just be trying to get back together. He returns in later seasons and is then able to further reflect on what went wrong and amend properly.
Finn takes his dating struggles as growth points, which becomes more obvious when you get into the later half. Bubblegum was an ideal, but the age gap was very clearly a hurdle, and I like that the show made a point about how there is an inherent difference between an adult and a child that makes the relationship infeasible. FP was Finn's pre-teen years of not understanding relationships, and he came out hurting the person he cared for. It was used again to show Finn hasn't fully developed yet and still has a lot of growing up to do. All this then culminates into Huntress Wizard, where Finn is now a more full person able to operate on slower, more healthy terms.