I'm sorry you are having a hard time. I have had lots of jobs because of my bipolar (I would get fired or quit) and have never told about my bipolar/mental illness issues. I really don't know how companies respond to hearing this. It's worth googling about it. If your bipolar is that bad that you miss lots of day/come in late, I would consider telling them. For me, I'd think being fired is worse than maybe vaguely telling your HR department or boss that you are struggling. Being fired sucks and it looks like crap on a resume. I know.

Good luck!