For me, being bipolar feels like a bungee cord. One minute you're having a great time, full of adrenaline, weightless, and on top of the world- and the next, you're plunging at the speed of light and hoping the cord doesn't snap on the way down. The feeling of the depressive half is so bad for me. I'm bipolar 1 with psychotic features, and I handle my mania really well- however, when it comes time to be depressed, it is so overwhelming that it physically hurts, it feels like my body is on fire, and it is hard so to do anything in that misery, even just move.
Other symptoms I struggle with are the paranoia and delusions when manic, the sleep cycle changes, and the absolute loudness of my brain. After taking medicine, I realized my internal thoughts were so loud they affected my processing ability- for example, I didn't know the clock on my wall audibly tick-tocked.
But to be honest- The hardest part is the isolation you feel, in a world where not many people feel emotions as strongly as you do, it is hard for them to empathise with how it feels to be in the middle of it constantly. I wouldn't say medicine would fix everyone, but what I realize is that my support system is great until my emotions veer too strongly one way or another- it didn't help me cope better, but it helped my support system empathise more. So when I'm in a crisis, and I'm back into that realm of "too far gone", it's still as bad as ever. It can feed into my paranoia and delusions that people don't really care for me at all, when I realize not everyone has the scope or experience to help people with my illness. It is still very isolating feeling, though, and somewhat aimless, like pushing a boulder up a hill to have it roll back on me again.