Feeling like a burden at work, why?

You're not alone. I often feel this way as well. I think that sometimes people who don't have bipolar disorder and interact with those that do may feel empathy but not fully understand the struggles, and this crosses over into people's behavior towards someone with bipolar. Now, that said, anyone that purposefully makes you feel inferior for having any mental health issues when you are causing no harm is not worth your mental energy. I know that letting go of people's opinions and stigmas of you can be hard, but it can also be very freeing.

I'm sorry honey. You are not a burden. My MIL keeps telling me how bipolar disorder is no joke and if I do this or that it will turn me into a psychopath because I'm already bipolar...

I'm sorry that's happening to you. It's happening to me too. I don't get it. But know you're not alone.