Hi, I'd like to help you with your question.
It depends on if you feel the new relationship is a good one for you. Are you in love with your current boyfriend? Do you see a future for the two of you? If so, then you probably should continue with your current boyfriend. However, even if you do not feel your current relationship has a future, going back to your ex may not be your healthiest choice.
What you need to focus on is your ex's behavior. In the past, he has been in and out of the relationship with you. Then he leaves without talking with you. He has made promises and not kept them. And he has promised to become sober and has not followed through. These are all indications that he is not reliable and not trustworthy. If you decide to go back with him, trust is going to be a big problem in the relationship already. And trust is the foundation of a good relationship, so you would have to deal with this issue.
What you may want to consider is letting go of the old relationship and letting yourself mourn it. Whenever something ends in our lives, we need to let ourselves say goodbye. This usually means we go through a process of grief and that is ok. It is how we cope.
If you cannot let go of your ex, you may want to consider therapy. There could be something about the dysfunction of the relationship that appeals to you. It may be because of problems in your childhood especially unresolved feelings from neglectful, abusive, or emotionally absent parents. Whatever you feel that attracts you to a harmful relationship with your ex is what you need to work out. A therapist can help you pinpoint the issue and resolve it so you can move away from the relationship and have better and healthier relationships.
I hope this has helped you,