I'm in this exact situation. We run a niche software product in the telecoms industry, and we have one major customer who is also the most opinionated on how it should work. They keep sending in feature requests (for the old, poorly written application), 'we' (i.e. the boss) keeps saying yes. I'm still here and there working on features that we apparently agreed to build for them over two years ago. At least the boss is learning to say no more.