This is all so new to you, so it is understandable that you find yourself panicking, fearful, second-guessing yourself, etc. We have all been there, and then some. I don't know your situation about children (if you were trying to have, or couldn't or whatever --- this can be a sensitive issue) but, it's one time in your life that you can be glad you don't have to put any through this.
So, go sip a margarita, have your hair done, etc. (what everyone else said). The bills aren't due now I am sure, so give him the benefit of the doubt of getting the money back into the account. If he doesn't, then you can start doing something about it. If you had children, I would've done something more immediate, but since there aren't, I would wait and see what happens. Give it until the end of the month.
There is a possibility of an OW because, generally, men don't leave unless there is one waiting in the wings. But, it seems it may all be about addiction to a new, destructive lifestyle. If he is living at his mom's, I am sure she will eventually find out what's going on. Until then, best to not involve family or friends.
Practice "letting go". As others have said, you cannot control anything or anybody except yourself. You can try and influence, but at this point, I doubt he would listen to anything you have to say. He left, "made up his mind", and trying to convince him of the error of his ways, is a waste of time now. If he could get to the point of leaving, he could also find his way back. So, step aside, give him his space, while protecting yourself financially as much as possible, work on yourself and your issues, giving yourself the space to do so.
Notice all the "you's"? Take this time to look inside (and outside) and work on those things you feel you need to. Find time to exercise, eat healthily, meditate to clear your mind, pray to connect to your spirit (if you believe in a Heavenly Father), hang out with friends and family to have fun.
Sorry so long --- take care.
Me:57 H:52 M:28 Got another lawyer last year and filed. D35,S/D twins28,D22 EA4/04 End? Who knows? "Life is like a mirror. Smile at it and it smiles back at you." — Peace Pilgrim