Saying 'Yes' when you mean 'No' is a sure-fire way to build up the stress and is one of the most common issues that women raise with me when discussing assertiveness.
You were not put on this planet to please everyone else, at the cost of your own health and well being, nor will the whole world think badly of you because you no longer say 'Yes' to everything and end up feeling guilty anyway.
Assertive behaviour means saying 'No' and backing it up with an explanation if you wish, but it doesn't mean making excuses, apologising or justifying your actions.
Spend a few minutes thinking why you have difficulty in saying no...
Here are a few examples of how to decline in the right way, for you and others.
Your friend asks for a loan.
You - "I have a rule that I don't lend money to friends" This demonstrates that you are not singling them out.
A colleague asks for £10 contribution to a gift for someone you hardly know.
You - " I've never really spoken to Fred, I'll just wish him happy birthday instead."
Colleague asks if you can give them a lift to the station.
You - "No", plus an assertive explanation if you wish - "I'm picking up my daughter tonight" All of these assertiveness responses should not be accompanied by excessive apologising, which just sends you back into passive communications.
It is worth while listening to how others say no, particularly noting those people who do it well.
I thoroughly recommend watching a YouTube video of a Pantene advert that asks - "Why are women always apologising?"
You can click over to it here.