Much of what I am about to say will reiterate and reinforce what many others have said.
First of all, unless he has been diagnosed with DID, do not be quick to use that diagnosis to "explain" his behavior. It is very common for emotionally (and physically) abusive people to claim they do not remember. They believe if they can get their partner to believe they do not remember then they do not have to take responsibility for their behavior.
Look at his behavior from another perspective:
He goes out drinking and drugging. He is physically absent. When you say something to him he explodes. (Or he explodes and then starts the rest of the behaviors). He comes back, tells you "I don't remember," turns on the charm, and all is well ..... until the next time.
This is the cycle of emotional abuse and violence, plain and simple.
He is lying to you so he does not have to take responsibility for his behavior.
He is manipulating you because at the end of his bull****, he "makes nice" and all is happy again, for a little while. He smiles, turns on the charm and gives you hope that things will be better.
You are stuck in the cycle of abuse.
The way to get out of the cycle of abuse is to say you are no longer going to tolerate it.
If you want to be in a relationship with him, you must create boundaries. Make them clear to yourself and to him.
Decide what it is you will and you will not tolerate. Tell him which behaviors are acceptable and which are unacceptable. Make things crystal clear, with no ambiguity or shades of gray.
Make your bottom line firm. Tell him what the consequences are for violating the boundaries you are creating. Make sure your bottom line is something you will actually do. If you do not intend to hold true to your bottom line, he will know he can continue to manipulate you.
If you do not have a therapist I strongly suggest you get one.
If he is not in therapy you may consider making that one of the conditions upon which the relationship is contingent. Your choice, but something to consider.
Setting boundaries is not "controlling" him or treating him like he is a child. Setting boundaries is necessary to keep you safe. It lets him know what you will and will not tolerate. It also gives him a sense of security. If you did not love him you would not bother setting limits on his behavior. He now knows where the limits/ boundaries are. He no longer needs to test them. He will know if he tries to push beyond what you have set forth there are clear and unequivocal consequences.
He no longer has to ask himself "how far can I push before she tells me she doesn't love me?"
If he chooses to test your resolve, he will find himself alone.
Boundaries are not meant to harm or humiliate. They are for the protection of the individuals and of the relationship. It lets each person know where they stand and it clearly spells out what will happen if the limits are violated. It takes all ambiguity out of the situation.
"I didn't know what/ that would happen!!!!!" is something that can no longer be claimed.
Boundaries will also force him to take responsibility for his actions. He can no longer play the victim "I didn't know what would happen," or claim he didn't "remember" what he did. Every choice has a consequence, whether the consequence is a good result or a negative result. It is his choice which results he wants.
The fact that he may have arrested emotional development as a consequence of the abuse is not an excuse to never grow up.
Sure, there are things that are more difficult for him, and one of the things which is very difficult is emotional regulation. He will learn, but not at your expense.
Since he has slept with others, please get yourself AND him tested (especially after this trip). You do not want any nasty surprises.
The drugs and drinking are his way of numbing himself from the thoughts, memories and feelings. They are his coping mechanisms. They are also part of his excuse for "not remembering," whether it is sleeping with others he claims to not remember, his emotionally abusive behavior or his being away without contact. He needs to develop healthy coping mechanisms.
Of course, you need to decide whether you want to be in a relationship with him at all.
You have periods of separation and he "calls you back for coffee" and acts like nothing has happened.
What he is doing is calling you to meet HIS needs.
He calls, you jump to answer the call.
You want to discuss what has happened since the last time you were together, he conveniently "forgets."
He is manipulating you. He knows you will always be there when he calls. He also knows he can do as he pleases, act as he pleases, and you will always be there.
You are NOT his or anyone else's doormat.
IF you want to be in a relationship with him, you are going to have to draw on every ounce of strength you possess to create and maintain the boundaries so that you and he can have a relationship worthy of the two of you.
If you feel this is too much effort, be honest with yourself and get out now.
NO ONE will think you have "failed," and neither should you.
You deserve to be in a relationship with someone who will also be putting forth the effort to make the relationship work while at the same time working to heal from his past.
Be gentle and kind to yourself.
Acceptance on someone else's terms is worse than rejection.