Firing a manager.

I iwn a busy, thriving consignment atore. I have a manager who Is excellent at administrative skills, but does not have the best people skills. She has had a couple of issues with employees under her, and I have been receiving customer complaints every few months or so about her. I have had a few conversations with her about adjusting her attitude both with employees under her and customers. For a while she gets it and everything run smoothly, but then a complaint crops up or an employee issue crops up. I recently had another customer complaint, and though I like this manager very much and get along with her on a personal level, I feel it is time to let her go. The problem is she is one of only two managers, and if I let her go right away I will be short. So I need to train one of our employees to be her replacement, and I do have a very good qualified candidate for that. My question is, should I talk to her about this most recent complaint, or just go about training her replacment and let her go once the new employee can take over? And then theres the whole issue of firing someone who has been with me for a few years now…I’m scared and nervous. It makes me wonder if I am really cut out to be a business owner. I know people who can fire people no problem, simply by saying it’s just business. I have done models already on tbis and i know its my thoughts creating these feelings. Thanks for your guidance.