I am sorry to hear that you are going through so much turmoil with your boyfriend. Yet, I think there are a few clues to the answer embedded in your question. There are some specifics about your boyfriend that I think maybe important to understand. The first is that this is a long-distance relationship. This in and of itself is important to fold into the mix. The relationship began and was supported by the fact that you were not together. Believe it or not, a long-distance relationship may have value simply because it is long-distance. The connection itself may rely on the two of you not being constantly available. This happens more often than you might imagine.
It means that each person may desire more, but actually not be able to manage more intimacy. There are many reasons this could be, but each person can show that they want the other while each remains unavailable. The norm is wanting while not having and this becomes the way the relationship stabilizes.
The fact that March was the time he was coming back for good may have served as a disruption to the stability of the long-distance rules. Very often this is when one person in the relationship does something to sabotage the relationship.
Secondly, there are a variety of symptoms with ADHD that can make relationships difficult. Here are symptoms of ADHD taken from this terrific article by Psych Central’s own Margarita Tartakovsky, M.S.
When an individual with ADHD becomes an adult the main issue often is impulsivity as elaborated on in this blog also by Margarita Tartakovsky, M.S.
If you put together the long-distance relationship with the fact that it was about to change in March, and combine it with the primary concern of impulsivity for adults with ADHD you have some possible explanations for why your boyfriend might be so attentive during the intermittent times you were together, and why he would have made an impulsive sabotaging decision to cheat on you just before getting together permanently.
But the more important thing than trying to figure out why your boyfriend did what he did is to make sure you learn lessons from this relationship that you carry forward. If you are looking for a relationship where the two of you are available for each other regularly then part of your decision making may be to look for someone who can be more present for the relationship.