Good question, and I hope others will take the time to offer their thoughts on it.

My thoughts:

If you give her too much, she's not likely to realize that her life can be better with you.

If you give her nothing, she'll resent you and be less likely to ever want to come back.

I'd give her just enough to stay afloat and let her know you still care. The x-mas bonus/gift also sounds reasonable.

Only when you are ready to give up, do you totally cut her off.

Maybe ask her about the car loan and she what she says, and decide from there.

M: A really long time.
Crisis: 5 years.
She's still worth it.

Life is never made unbearable by circumstances, but only by lack of meaning and purpose. -Viktor Frankl