My boss and human resources presented me with a performance improvement plan. I responded that I disagreed with the allegations but agreed to attend leadership training course. I now don't trust my boss or hr to have my best interest. Can I ask for a different "mentor" in this process?

Did they say they were unhappy and you could either resign or be fired OR take the course? I don't think I'd rock the boat. If they are unhappy with an aspect of your performance they could very well simply fire you. At the moment they are looking at you for improvement in their eyes, whether justified or not.

It has been difficult as I feel I was betrayed by my boss who did not bring any of the issues to my attention before going to HR. It was a complete shock. No other employee has ever filed any complaint against me its just my boss. I asked HR to investigate the claims they said they did not have the resources and just basically sign the document and move forward.

I know this is painful to hear but maybe the best (and only) thing you can do is as they say - just grit your teeth and move forward.