Dating with mental illness

I hate having depression and bipolar disorder. It's hard enough to function day-to-day but dating is especially difficult. I'm afraid to talk about it because I don't believe anyone, especially the person I'm with, could understand the severity of it. I know he'd be supportive but I'm just so ashamed and embarrassed! Honestly I've never told anyone. I take medication but even then I have slips of highs and lows. What kind of twenty year old am I to have such immature issues? I just don't know how to tell my boyfriend. As of now he's believed I'm just moody and I constantly have a lot on my plate 

I'm so sorry this isn't heavy hitting and packed, I've never expressed talking about how it effects my social life. I just needed a little vent