not sure you should place all the "blame" on his choices on addiction. look at the FACTS. of your short 3 year marriage, he was involved with someone else for close to TWO YEARS - the 16 months you've been separated and for months before hand. he left you and the kids, went straight to her and has been with her ever since. he no longer is in contact with his OWN child and now she has lost custody of HER own kids. they are engaged. he filed for divorce. he left the state.
those are his choices. he's gone dear.
due to your still conflicted feelings on the matter, it's probably advisable to cut ties with new dating person. are you going to meetings and in counseling? those things can really help you work thru things and sort it all out.