How can I let people around me tactfully know that for the foreseeable future, I am done helping anybody with anything?

I know the title probably makes me sound like a selfish *******. Why wouldn't I want to help people around me? Well, here's the thing. I've been kinda used to the "helper" role my whole life. And just recently, I have kinda come to the conclusion that I don't want to be that guy anymore. People rarely text me or want to hang out with me just because we vibe well together (many thanks to those who do), it's mostly just because they need something from me or need me to come and fix something for them. I've always said yes to helping others when I knew I could, hell it used to make me really happy. But I am starting to see that most people are never thankful and never repay the favour in any way. I am starting to feel used and I don't like that one bit. I want to be that guy who people text and are like "Yo, let's have fun, let's go to XYZ and have a good time." Not the kinda guy who people text with "Yo, my easily solvable problem would require me to use my head for more than 10 minutes, so come and fix it instead k thanks." I am just utterly done. My career and education require a lot of time and energy from me and I need to spend it there, not on other people's issues that they can easily solve. I have promised to build a computer for one of my best buds next weekend, but that's it from me for at least half a year. How do I let people know? Should I just start turning down all the help requests? Should I even explain why? I feel like it is well within my rights to use my time for myself, especially when I never (or very rarely) ask anybody for anything. But I still can't help but feel like an ******* for typing all of this.
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TL;DR: Emotionally drained from helping people around me, need to know how to tell them I'm not gonna do this anymore


Source.