Get a solicitor, get the date of separation officially noted and start working on the financial split.
I've both experienced this and seen this before.
A woman with drive and ambition gets together with a less successful man (by society’s standards). She potentially has a house, car, more successful career etc and he rents/is out of work/on minimum wage.
She lifts him up with financial and practical support. Rewriting his CV, coaching him through interviews and applications, training him in a task/profession he doesn’t previously know; life looks to be on the up.
Then the man, when out in the real world, starts realising that people look at him differently. He’s noted as the smartly dressed, successful, confident man with the nice house and the nice car and the nice standard of living, and people start treating him differently. There are admiring glances, hints of jealousy, praise and admiration.
But at home, with his partner, they treat him the same because they’ve seen him in the rough times, they’ve bailed him out, been his teacher and his comforter.
The man starts preferring the admiring gaze of strangers, to the knowing gaze of his partner and he moves on without her, preferring the ego boost of the new world he’s now a part of.
i believe that’s what’s happening here.
You won’t get him back (unless he falls on his arse and needs you back to rescue him). He wants someone who sees him as the teacher, not the student.
In short, he’s used you to get a step up and now he’s on that higher level he wants someone who never knew he was on the lower one.
There probably is another woman as well, even one that’s just been flirting with him, rather than an outright affair. He’ll have told her that your success is down to him and he’s a self-made man and blurred over your assistance in his growth completely.
It’s more common than you think.