You are an endless source of surprises. 21! You must be one of the youngest, if not the youngest around here. Which should tell you that you have achieved more than most of us in terms of waking up and acting on self-destroying behaviour, allowing you a full life to do incredible things.
You have also had 2 kids when most people don't know how to look after themselves. You can look after yourself and others (including your partner). At the moment you are the equivalent of a high flyer broker who is running around keeping all the balls on motion, under huge stress, because the markets don't close. Your market not closing are your young kids. Don't see yourself as a mother who cannot cope. From the little we know about you, you have far more skills and resilience than most human beings I have ever come across, let alone addicts.
Your partner has abandoned you and his kids. The fact that you understand his addicted mind and can empathise with it, does not change reality. It is a **** thing to do. I don't know how, but instead of worrying about him, find it inside you to get angry and detach from a problem you cannot resolve.
I keep saying the same, but reach out: explain to people you trust what is happening. Explain that you may still stick by his side even if you know it is a bad idea. This is not about disappointing your parents or friends. It might happen because addiction takes everybody with it.
Do you know that statistically, women who have children this young, do better in their professional lives? You have a full life ahead of yourself, the wisdom and strength to make it happen. You seem amazing. Would be great if you kept posting.