There’s the whole ‘broken trust’ aspect, from the financial stuff. People don’t want to have sex with spmeone they can’t trust. Maybe she is also exhausted from working and taking care of your four children. Perhaps taking some load off of her by helping out more around the house would decrease the stress levels and make her more likely to be in the mood.
Two years is a masters, not a PhD. Still worth doing.
A PhD thesis is approximately equivalent to someone working full time on a specific subject for four years, if you include coursework and such into the time.