I've looked up some more about McGregor which made things even harder on McGonagall. Her love interest McGregor would eventually marry a muggle woman, after McGonagall turned his proposal down due to her career at the ministry of magic. In 1970 however, McGregor was murdered by Death Eaters in an random muggle hunt, and she has ever been wondering if she would have been able to protect him if she was his wife, feeling guilt ever since. It appears this was all revealed on Pottermore.
Of course, for Dumbledore, who was blinded by his lust for power and love for Grindelwald, his wake up call came when his sister Ariana died, and so his love also left him with sense of eternal guilt. In "The Crimes of Grindelwald" we can see Grindelwald appear in the Mirror of Erised when Dumbeldore approaches it, implying he still loves him. I wonder if the Fantastic Beasts series will reveal more about this awkward relationship.
Now McGonagall in eternal guilt, Dumbeldore with eternal guilt, and yes Snape too, since he told Volemort about Trelawny's prophecy leading Voldemort to kill Lilly, and Snape too never forgave himself for that, and protecting a boy he loathed (Harry), was the only way he could make it up to her.
So in the end, three Hogwarts teacher whose lover didn't bring them happiness, but feeling guilty for the rest of their lives. Yes, love is wonderful, but also terrible, depending on how things work out.
