The End of it All
She ended it after Hagrid was expelled – he knew that she had suspected him of framing him – she was always too perceptive for her own good. But that was one of the things that had attracted him to her in the first place.
Yes, Hagrid's expulsion sounded the death knell of their relationship – not that it would have lasted much longer, unless she had agreed to be his consort. If she had, then he would have made her immortal as well. Her skills would have been an enormous asset to achieving his goals – unfortunately, it didn't work out that way.
At times, he even regretted framing Hagrid – it had cost him Minerva. She always had a soft spot for Hagrid. But he had to – there was no other option, though he wished there had been. It was the only way.