I think there's a good way of phrasing it that doesn't hurt anybody's feelings and ensures your partner doesn't feel betrayed: say you lost interest and motivat

I think there's a good way of phrasing it that doesn't hurt anybody's feelings and ensures your partner doesn't feel betrayed: say you lost interest and motivation in the product and at this point you're more of a liability to the company than an asset. Say you don't want to bog down the progress of the project and that it's in everybody interest that you move on and let people who are truly passionate about it give it their best.