Does your MIL live with you?
I don't think SAHM gets worse, I think that you are going through a bad period and it can only get better.
See....I guess my worry about be that things would be WORSE if I went back to work in your situation. I would think, either my child will be ignored and develop poor behaviors in daycare, or if my MIL were watching my child during the day (and I had issues with her), then I'd think there'd be a million ways she could annoy me that way, and if DH and MIL aren't helping as it is, they are still probably going to be expect me to do everything when I get home from work.
I'm sure ALL SAHMs have rough periods on occasion. Perhaps a family meeting might help, complete with a chart of "jobs" for everyone, to help alleviate your stress.