You are not helpless in this situation. This is not a personality disorder. You are unhappy in your marriage but you don’t see a way out or a way to make it better. So you are obsessing about this man as a possible rescue. I do understand why the fantasy of going back in time and picking up where you left off is so appealing. I do sympathize that your life feels so bleak right now. But you can’t erase 11 years and 5 children.
You are absolutely right that what you are doing is irresponsible in terms of your children’s welfare. What will happen to them if you somehow manage to ride off into a sunset with your old love? You’re going to leave 5 kids with a heroin addict??
Once we have children, our lives aren’t our own. Your first concern needs to be with them. If your marriage is truly unsalvageable, then do something about it. If your ex feels as strongly as you do, he’ll wait until you get a divorce and he’ll embrace your children. If he’s not willing to do that, accept that you are letting yourself get swept up in a fantasy that is certain to fall apart as soon as you try to make it real.
If reality means staying in your marriage, then please do what you can to make life better for yourself and the kids. See if your husband will get help for his addiction so that he can be a better husband and father. He brought these children into the world with you and he too is obligated to do what is right for them. If he can “divorce” his addiction and turn his attention to his family again, you may find that what you once loved in him is still there. Meanwhile, consider joining an online community of wives of people with addictions or reaching out to people who love you. They can give you some much needed support and practical help.
I wish you well.