Very confused- Fiance has ADHD -a roller coaster relationship

I am not sure this is being post in the right forum. My fiance has ADHD and we are in the midst of another break up/make up cycle. I love him and am not looking to change him, however I am becoming weary/anxious about moving forward with things with him, the instability of the relationship is making me question whether we are  strong enough together to work through whatever obstacles that we have to face.

One minute he is sweet, charming passionate and loving. The next he goes into withdraw mode where he barely wants to communicate, we have our own places right now.  He even ignores communication efforts by me when he is in this mode. I don't ask or push for too much communication for fear it will drive him the other direction. I did gently suggest though to sustain the relationship that we do need to maintain some level of communication (even bare minimum when we can't see each other) just to check in. At times from his reaction, this seems too much for him.When we have discussions that become heated, he has zero filter ( I have learned to let the mean stuff he says roll right off my back, it is not easy though)  and wants to shut down the discussion most times in the middle of it so we basically never resolve what the issue that lead us to that point. He goes back and forth between believing in the relationship and wanting to back out of it. But then when he ends things he will turn around a day or two later and question me why I stopped coming around or checking in with him and questions my commitment to him. If I remind him that he ended things he turns around and says he he no idea what I am talking about. I feel like I need to tape record half our discussions to show him I am not making this stuff up.I love him dearly and am a patient person. I don't mind the messiness and cleaning up after him or the forgetfulness. I am just a little unsure of how we will fare in the future if we can't figure out how to approach these relationship issues now.