Jo you're in a tough spot, but please don't let your thoughts run away with you. You have no idea what the other woman is like, but if they met in rehab and your AH is drinking you don't it will probably all end in tears. Give it time and keep your mind away from it for your own good.
You really do need some outside support. Do you have family or friends who can help in practical ways? Put you in touch with agencies who can arrange financial and legal counselling? You will come through this. Have pride in being the mature adult, there for your children.