Sorry to hear about this situation.
I'd suggest not beating yourself up regarding the why's and how's of being with this man for so long. I wonder how I ended up with an AW myself and it's just not a positive thought. Things happen for a reason and I'm personally allowing some HP to run the show. It takes a bit of weight off my shoulders and maybe it will for you too.
I just consulted with a lawyer yesterday. If you have the kids and he left the house, then that's considered abandonment and won't be looked at favorably for him. Therefore, I'd recommend you start doing a little google searching for a divorce attorney. I was referred to a heavy hitter and even he does free consultations. My guy has given me just over 3 hours of free consults.
Some people here have already mentioned that you can do it yourself. I know it's a pretty dark space that you're in. Keep your eyes open for the glimmers and move on them.