Hi, MPC. Welcome to the forum.
It's a bit odd you'd choose Mrs Something as your screenname when you're thinking of leaving your husband.
It's great you're dreaming big and want to spread your wings now that you're in your 40s and wanna live larger than you have been. However, you know real life isn't erotic literature... really making poly work is nothing like a sex fantasy. If fantasy gets too "thinky" and there are tears and self doubt and jealous rages and sulking, it kinda kills the sexy feelings you're striving for in the fantasy.
If your marriage is really on the rocks, focus on getting out, getting a new apartment, dealing with your kids' feelings, employment/money issues, etc.
If you then have time to also pursue new relationships, go for it. I separated from my ex in Sept 08 and met a new partner in Jan '09 kind of by accident, but my kids were all older teens or in their early 20s at the time... I still have to juggle their needs as well as my divorce proceedings, money issues, etc.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
me: Mags, 59, living with:
miss pixi, 37