Hey, I used to be like you but a good friend at work taught me how to change.
I learned to say , "I am honored to be asked to help with your project but at this point I have to decline do to previous committments.
You do not have to explain yourself, just say no in a polite and assertive manner.
You can learn to do this and then throw away the guilt you feel for saying no and don't change your mind.
Good luck and gentle hugs