Regretting involving a friend.

I am starting a new venture and I thought it would be a great idea to involve one of my closest friends as she could contribute so much. When I asked her she was excited but as time has gone on (only a few weeks) I’ve noticed her negative attitude coming forward. I already knew she could be like that but I was hoping she would be able leave that to one side when it came to work. Each negative comment I have managed to help her see the positive alternative but I’m finding it so incredibly wearing and I’m regretting involving her – to the point I don’t want to work on the venture as I don’t want to work with her. I really don’t know what to do as ideally i’d like to go back to this being my venture without her involvement but I don’t want to upset her. I want to ask her to not be involved but don’t think I can after asking for her help. Any advice?!