That little word can be hard to get out. We like to think of ourselves as agreeable people, easy to get along with. Saying 'yes' seems to be part of that. 'Yes' is a perfectly good word and you should make it part of your vocabulary. When new ideas and opportunities present themselves, don't say no automatically. Maybe you'll like river rafting, camping or sushi.
But conversely don't be afraid to say no. Say it politely but say it definitely. I had a recent occasion in my local bulk barn store where I had the opportunity to exercise my right to say no. I should keep a list of unit prices for the items I purchase regularly. It's on my to-do list (sort of). When I buy chocolate chips in a package I could easily make a note of what they cost per 100 grams. The reason to do this? So that I can tell if buying the item in bulk is a better deal. Unfortunately, it's not automatic.
On one recent shopping experience my chosen items were going through the checker. I use small jam size mason jars, getting them weighed first so that their weight is taken off my cost of the item. Sliced almonds went through and a price of almost $10 was rung in. I was mildly shocked and without hesitation said 'Stop!' I think I scared the new trainee as his fingers jumped off the cash register. I continued in a calmer zone and told him politely that I wasn't expecting the almonds to cost that amount and I didn't want them. Now, being new, he didn't know what to do. I guess I should have been embarrassed as they had to come out of my mason jar that I had ladled them into. They didn't really have the tools for the job.
I had a polite conversation with the more experienced clerk who didn't seem/didn't act fazed by my request. I told her I wasn't expecting the almonds to cost almost $10 for a small jar. I made some comment about the cost of a bag of sliced almonds at the supermarket and how it wasn't that expensive but I tried to let her/the store off the hook by saying that I should have checked the weight myself.
Somehow I suspect a younger me would have felt too embarrassed to speak up. Have you ever felt that way? I guess it's a little like having to put back items when your bill is more than the cash you have with you. There was a day when supermarkets didn't take plastic cards. For me, I can afford the $10 but it goes against my principles. That would be my principle of not feeling ripped off.
I remember more than once in leaner days ordering something less expensive when out for a meal with friends/colleagues. Some thoughtless/calculating person who just happened to have ordered the equivalent of steak and lobster comes up with the idea of splitting the tab, 'to save the hassle for the server.' It still annoys me that I went along with it, not wanting to feel embarrassed. It coloured my relationship with the glutton though. It would have been better if I had just said, "No, I prefer to pay my own bill separately."