DC is not a place to look for relationships right now. The current regime doesn't like them, even the historically strong ones.
That said, currently I can't think of any and while yes they don't work out, it's not just that specific pairing. Generally, superhero relationships go on for a while then die until someone gets nostalgic. Clark and Lois are me the exception than rule.
For Mr. Terrific specifically, I don't know that he's built to have one. Other than being the third smartest man and tech, he's a widower and atheist. That's it. I'm oversimplifying, but he has no rogues or supporting cast of his own so developing him in a relationship actually ends up making him more generic because he loses an element and gains very little. Sasha was a good direction if I recall because she was a natural fit in his universe (over on Checkmate), their skillets put them in places where he could shine, and he could grow.
He's doing well over in the Terrifics and I'm hoping he's coming back to the JSA as well, but I don't know if a relationship will ever work with him.
Power Girl is a common factor between Val and Michael but both times it seemed like they had no idea what to do with either and nobody was well served. The PG characterization most tend to respond to is the sassy lady who takes no shit, and stoic doesn't pair well with that, so perhaps that's why these failed outside poor execution. I imagine she needs a more dynamic love interest. They usually try to hook her up with more series heroes, maybe someone more upbeat would land. I don't know.
Watch Justice League for DC's best relationship of that nature, but speaking frankly, superhero comics are a bad place to see relationships work out. Even the evergreen ones (Ollie/Dinah, Arthur/Mera) get separated all the time. Hell, even Lois and Clark do from time to time.