The good news is that you are not alone and you are in good company. The great Russian composer Tchaikovsky (The Nutcracker, Swan Lake, Eugene Onegin, etc.) had this problem, too.
An opera singer Korsov kept bugging the great composer to write an aria especially for him. Tchaikovsky was a very sensitive, polite kind of person. He was incapable of refusing anybody directly and decisively. Korsov knew that the composer would never refuse him in strong terms and kept insisting.
Once, he appeared at the composer’s place unannounced. Tchaikovsky’s butler had been instructed to lie to Korsov and tell him that the composer was not at home.
“Never mind, I’ll just wait for him in his study until he comes back,” the insistent opera singer said.
Tchaikovsky heard that from his study and panicked. He didn’t want the obnoxious singer to find him in his study and discover that the butler had lied to him. So he hid under a sofa.
The singer came into the study, sat on the sofa, and remained there waiting for Tchaikovsky for over three hours.
In the meanwhile, the world-famous composer stayed under the sofa, almost suffocating and trying hard not to sneeze.
And after that ordeal, he actually gave up and wrote an aria for the pushy singer.
Not being able to say “no” is not a character flaw. It is actually a sign of sensitivity and tactfulness that pushy people who can’t get a hint often exploit. Don’t let them think that you are the one with a problem. If an adult is incapable of making a request in a way that makes it very easy to refuse and fails to see one’s unwillingness to do what s/he asks, s/he is the one with a flaw.
It just bugs me how often people who are sensitive, tactful and kind are pathologized. What’s up with all those self-help books that teach people how to stand up for themselves and say “no”? Where are the self-help books for bullies and obnoxious people? Is the solution really for all of us to learn to bully others? Is the ideal society the one where we all turn into the worst stereotype of a used car salesman?
And I bold-typed the preceding two paragraphs because I think that this is a very important point.