I'm a nice person but I think I'm done.
How do you think no longer being nice would make you feel.
Like a cloudless blue sky.
Are too many people asking you for help and other things from you? It's good to be nice but if there is too much pressure on you to help all the time or if people don't appreciate your help then it is justifiable to take a break from them. Otherwise, people will just keep taking advantage of you.
That's exactly why I'm through. Now I'm going to be mean.
While you should definitely not allow others to take advantage of you, it's wrong to be mean towards others. Trying being assertive and direct with people. It's effective while still being respectful.