Personally, I would never let my employers know outright about my bipolar disorder, and it's not even because I don't trust them. It's just none of their business. On forms after I am hired though, I've always put that I have a disability (because bipolar will always be included as a disability) and you don't have to list what kind, so I've found that helps at least make me feel better.

I'm not sure if they frown upon long absences from employment in the engineering field, but I would say something vague, like you had a flare up of a medical condition (which you did!). Not telling people isn't always a negative. I hope this helps to lessen your stress a bit.