You have only been in this relationship for 3 months. My advice? Cut your losses and move on. From what you wrote, it looks to me like he has never divorced his wife and never will. The two of them have an arrangement that suits them. He has protected his children by leaving but has also stayed in relationship with their mother — for 10 years! That isn’t likely to change — and, in fact, hasn’t changed at all. He hasn’t been honest with you from the start. Your “connection” was based on a lie.
I’m sure this has been very confusing to you. You are only 20. I’m guessing from your letter that he is considerably older. Please don’t beat yourself up for believing a liar who has been practicing his lies for a decade. He has taken advantage of your youth and inexperience. Give up any fantasies you have that he can change and make yourself available for someone who can love you, cherish you and be honest with you, as you deserve.
I wish you well.