Hi PeacefulCareBear,
Sounds like you are just starting to recognize the issues in your partner's behaviors. This is something it is very important to sort out! If you haven't looked yet, there are
Lessons on the right side of the board titled
"Understanding your partner's behavior" and
"Understanding your role in the relationship" that might help you as you sort these issues out.
When you say he's been "disappearing" what do you mean exactly may I ask? Do you notice any patterns to his mood swings?
Take care!

I have a long distance relationship with him and live three hours away. When I visit him, it's usually for one week or two weeks at minimum. He doesn't have many friends but he does spend a lot of time with one of his friends who comes over and visits. As for me he can only see me when I visit.
Anytime I do have to leave he does suddenly get sad. He won't mention it to me, but I can see it in his eyes. Anytime I end up going back home, he sort of retreats again and when I asked him why, he told me he likes space. So I think in my opinion he has so much different emotions going on so he sort of wants some time for him so he can feel a bit better. When I message him, I just tell him I'm thinking of him and to message me when he's able too. I never try to push him or anything because the more I message him, he'll sort of retreat longer.
He's been doing this each time I have to leave and eventually talks to me again after a few days. When I said goodbye, I did get a bit sad and cry and he did comfort me. Anytime I looked at him, he had lots of sadness in his eyes. The only time I feel he's at peace is when he's video gaming or we're both laughing together.
When the episodes flare up is when I see his cold and bitter side to him and I know I didn't do anything wrong but he's very angry and bitter. He won't even mention what I did, he just is completely silent and I always find myself something to do and leave him be.
I love him very much and joining this forum has truly helped me so far. His mood swings are not explainable, but rather unpredictable.
I have bipolar disorder and I'm very aware of all my mood swings and anytime I'm angry or anything, I always find something quiet to do or color or read a book. Other times I find taking naps to give me lots of peace. Once I always wake up, I feel better. When he's angry, I sort of don't engage with him. Instead I tell him to talk to me when he's ready and eventually he will open up and talk to me.
One day at first he said very coldly, "I'm not a talkative kind of person... ." When he said this I felt hurt but continued to keep doing what I was doing, than finally he opened up to me and told me how stressful his day was. So there is times where he snaps out of his angry mood and other times it's hard to get through to him or to even have a conversation. Instead I figured finding my own thing to do keeps me away from things blowing up and getting too extreme.
When we first dated, I just thought he was a moody person but his moods affect him deeply. Anytime we're finished talking, I always give him hugs.