I too struggle with saying 'no'. For me it stems from being afraid of disappointing people or getting judged. I find myself often overwhelmed as a result.

Therapy is having some success these days - particularly with the worry something bad will happen if I don't say 'yes'. She is giving me some skills to work on catastrophic thinking. Not sure how you could have better handled the specific example you gave. But when people have asked for help or my input I have been garnering the courage to ask them if they need it right away. That way it becomes easier to say that I can't do it immediately and they usually back off.