Holiday is canceled - so disappointed in my partner

We were supposed to go on holiday yesterday, but had to cancel it. My partner (who has ADHD) had terrible mood swings the day before we were supposed to go. He says it is due to a conflict with hos family (he has serious relation trauma and the family often lets him down). He got moody and angry towards me, and I felt so disappointed (this has happened several times before and we couldn´t complete our plans). We had long been looking forward to the holiday and I felt like we really needed something positive - after a year with relationship conflict and problems. He goes to anger management classes and has a difficult temper.

I am new to this forum. My partner is newly diagnosed with ADHD and we have a young child. Sorry for my English (it is not my native language).

My problem is that once he has these mood swings, I get into a difficult mood and I cant get out of that state. He suggested we go on the holiday anyway, but I couldn´t get into a good mood again. It felt like I had lost all hope and the good emotions about going away had been damaged - once again.

What to do? I am thinking of leaving him. But as I said we have a young child and he is a good dad (and he has some good qualities, apart from these bad ones). He says he wants to do better - but end up making the same mistakes over and over again. We had being doing better for a few weeks now - WHY start something the day before going on holiday? It seems like he is deeply destructive in some ways.

I am highly sensitive and his mood and anger affects me deeply. I also feel like I have no one to turn to (my dad is sick with cancer so I dont want to upset my family. I have friends but few of them are very close, and I don´t want everyone to know about our large problems...