It feels like you are asking for a secret quick fix that somebody here might know of - but it doesn't exist. You have to be upfront and honest. This is all part of what being an entrepreneur is about. The survival rate of co-founders is really low, and how it turns out is all up to how you handle the situation. Be upfront and honest and just get it done, don't let it drag on.

How you handle this will test your character. I know some people who are terrible at confrontation, but some of the best entrepreneurs and business people that I know are professional and just get it done - keeping it to strictly business and not making it personal.