Just a (real) question: why do you value other people's feelings over your own? If they love you they would not wish you to be uncomfortable, in which case they shouldn't ask. Once you've said no they will know the boundaries of what's yours and what's theirs and should respect you for it and appreciate they shouldn't ask again.
Practice saying 'no' it when you're alone (I know it sounds weird but it may help) and give a finite rule rather than justify it. Once you lend a bag yo whoever, you have in fact set a precedent, and then it will be even harder to say no next time they ask.
Suggestions:
"I'm afraid not, I like to keep my bags just for me"
"Sorry, it's the one thing I can't do"
"No, it makes me uncomfortable to say it, but I'd be more uncomfortable lending and I know you wouldn't want me to feel that way".
"No, not to anyone. Even you I'm afraid"
"No, I don't lend my bags, it's a rule I have"
etc
If they whine and moan and plead again, just repeat the same or alternative versions of the above. Having 'a script' means you don't have to think on your feet, justify yourself, or have nothing to fall back on.