The only problem is that I think I'm losing a certain amount of sales.
Collectors like to feel they're getting good deals no matter who they are or what they buy.
Many also have personal reasons for wanting to pay less such as limited budgets, high cost of living expenses, temporary cash flow problems, and so on. Asking whether a selling price is firm or offering less than full value is entirely normal. As you've discovered, if you get rigid whenever this happens, you're going to have problems. Collectors who buy directly from artists often do so because they like being around artists, visiting their studios, seeing how they work, and talking about art. They prefer this to the more sterile environments of galleries and gravitate toward artists who listen and respond to their needs.