Communication advice with SO

I get so frustrated with my SO sometimes. We've been together for a couple years now and we have a recurring issue that I'm not sure how to go about solving. I grew up in a household that made it difficult to voice my feelings. I continue to have issues with this now, though I'm trying to get better with it. Whenever I get upset about something and my SO doesn't understand why I'm upset, he expects an immediate explanation. Most of the time, I'm willing to give one, but I need a minute to compose myself some because I hate trying to talk through tears and just generally sounding even more pitiful. But most of the time, my SO gets angry when he doesn't receive a quick enough response, and his anger leads me to not saying anything at all since anger is still a trigger for me. I've tried to explain this to him, but he's bipolar and has difficulty controlling his emotions, so a lot of issues go unresolved and we're both left feeling hurt and upset. I'm not sure how to fix this issue and it's really starting to cause some serious problems. Help?