She doesn't speak English that well. Not terrible, but it's like constant grammatical mistakes that a 4 year old wouldn't even make. It drives me up the wall. She refuses to take lessons.

Hey, I am an adult ESL/literacy instructor in an English-speaking country. This is ENTIRELY normal and typical, even with students who have advanced proficiencies in English. You can take all the lessons in the world and still make basic grammatical errors because your brain is processing things from the perspective of your first language (it's called interference).

As for lessons, well, she's got 2 year old twins and a baby. Exactly how many women like that do you think mantain their studies/language lessons? Very few. Most of them drop out, I promise, it is NOT just her. Heck, I'm looking at my attendance sheets and I've 4 or 5 of them crossed out already this term.

She sounds like she's struggling with some depression. She's feeling isolated and insecure. You said her "body is wrecked" and you guys have grown apart. Can you blame her? The au pair thing is a sign of her insecurity.

Instead of an au pair, find an occasional babysitter and take this woman out on a date. Love her. Stop judging her. Start rebuilding her self-esteem.