The combination of ego and a strong desire to keep my word has had me struggle sometimes with people's requests, but one thing i'm learning as an adult is to feel better about saying No.
Typically, when I request a favor it involves reading between the lines - any response that's not an eager yes means a no. I allow the responder gracefully ease out of it without pushing, but not everyone works that way.
Some folks don't get the hint, so from experience going quiet or saying I'll get back to them piles on more pressure and doesn't close things.
Now, when uneasy about a request I'm learning to plainly say no even if I can't explain exactly why.
"I'd like to loan you the money you requested, but will need it before you can pay it back."
"Sorry, I got overwhelmed and won't be able to."
"I thought I could get it done easily but it's more complex than I imagined"
"Things have changed since i said yes."
I also remind myself to raise a white flag early and renegotiate if I can't continue as expected, and that this is way better than a broken agreement.