I hate saying no

I would like to start of, that I don't know how to say no to people, I have this ridiculous habit of always agreeing with others and not expressing my own opinion. Although when I do and when I actually turn down some plans/ideas I feel like the worst person in the world.

Not even sure why is this happening or why am I feeling so bad, but it's like people want to spend time with me/do some things and I turn them down.

Just today I got called out to do some photoshoot from my friend, he said that he wants to try some new lightning things etc, but I told him I'm too tired from work which I really am, I'm totally worn out but I could've taken an hour or two, to first help him and second to spend some good time together.

I don't know if this feeling is normal or not, I really hope it is.. Add some comments about how do you feel on moments you tell others no or can you even say no to other?

Do you care about yourself more than the others or the opposite?

Lets have an active comment thread going on here.