It sounds like even after 3 years of separation, you still have feelings for him, and it's really difficult to heal through the separation when both of you have to stay connected with each other for your kids. I'm really sorry to hear that...
You may need some more time to process your feelings and thoughts that emerged from the separation three years ago; Feeling the emotions from the break up, as well as accepting what has happened in order for you move on forward with your boyfriend. At times you may feel angry, or sad, or disgusted, these feelings are all valid and normal as part of the healing journey from a heart break. Feel your feelings, and find some grounds to why you're feeling this way. For example, you may be feeling angry from the disappointment of your ex not being able to fulfil his duty as a loving partner to you, and a father to his children. Or you may be feeling sad and guilty because you felt part of the break up was your fault, and you felt you could've done something better about it. The important thing to know is that, all your thoughts and feelings are valid, and you have the right to be feeling this way.
Without the emotional connection from your ex, it will be difficult for you to get any closures to the questions that you have in mind. You may need to find the closure through other means. This could be through a close friend/family whom you can speak to openly, or a professional service such as a therapist or relationship counsellor. They'll be able to listen to you, and help you through finding the answers/closure that you seek.
It's great that you have a goal of wanting to just be friends with your ex, so that the both of you can better support your kids together. I feel that's something worth holding up to in order to encourage yourself to heal through the separation. Perhaps you may need to find a way to limit the communication with your ex, so that you have space away from him to heal through your separation. You've done really well making it this far, and I'm sure there'll be better things ahead of you.
Take care lilykitten, happy to listen to you more.