You're not a good person. You're just afraid of conflict
Staying quiet, saying yes, and letting people walk all over you is easy to call kindness. Especially when you're terrified of saying no.

You said yes again. Even though you didn't want to, couldn't fit it in, and already hated the entire evening ahead of you. But someone asked, it got awkward, and your mouth said “of course” out of habit before you had time to think.
Now you're sitting there, angry. At them, at the circumstances, at yourself. Especially at them: they should have realized you were struggling.
Interesting communication system. You say “yes” and expect them to love you enough to hear “no.”
Sometimes people really are using you. Sometimes they know perfectly well you won't refuse, and that's exactly why they keep coming back. But sometimes they just believed what you said.
You said it worked for you. They assumed it worked for you. You said it wasn't a problem. They didn't go looking for a problem underneath your smile.
Then it turns out you've been keeping a ledger of resentments in your head for six months. Every yes recorded as a sacrifice that nobody has properly paid you back for.
But you haven't shown anyone the bill yet.
I think “kind” sometimes makes a very convenient cover for fear. A kind person sounds nice. A person who trembles at the thought of someone being displeased with them sounds a little less noble.
So you explain that you just don't like arguments. That you know how to compromise. That relationships matter more than little things. Except somehow those little things always get paid for with your time, your sleep, and your plans.
At what point do you get to exist in these precious relationships yourself?
Saying no can put someone in a bad mood. That's uncomfortable. They might be disappointed, argue, tell you you've changed. And this is where a lot of people surrender before the conversation even starts.
They need to find a way of phrasing the refusal that everyone will like. So nobody feels inconvenienced, nobody gets upset, nobody thinks anything bad about them.
That wording might not exist. Someone wanted something from you, and they didn't get it. They're allowed to be unhappy about that. You're allowed not to fix it immediately by surrendering.

And no, you don't need to turn into a rude bastard to do this. Some people discover personal boundaries and start behaving as though they've bought a license to be an asshole.
You can calmly say you can't. You don't want to. You're not willing to discuss it. You're not going to lend them money. You don't have to humiliate the person asking or lecture them about your new personality.
If you've already made a promise, you have to acknowledge it and deal with the consequences. If your plans have changed, let them know. Self-respect doesn't cancel out consideration for other people.
There are also situations where a direct refusal really is dangerous. If someone is threatening you, controlling your money, or might become violent, “just say no” is useless advice. First you need a way to protect yourself and get support, not an impressive argument on principle.
But if it's an ordinary friend who wants to take up your only day off again, check: are you actually in danger, or do you just not know how to sit with someone looking displeased?
Those are different circumstances. It helps not to call every awkward moment a catastrophe.
The worst part starts after the first refusal. You might feel ashamed. You'll want to send five more messages explaining yourself, offer to make it up to them, and eventually say yes after all.
You can do none of that. Let the conversation end. See whether the relationship actually falls apart or whether, for the first time, you've simply stopped performing your usual function.
Some people will take your “no” perfectly calmly. Some might even start understanding you better. And some really will disappear, because their interest ended the moment the free service did.
That's a shame. But at least now you know who you were actually dealing with.
Kindness you choose freely doesn't require you to hate someone afterward for what you did for them. Try helping someone that way for once.