Yes, it's okay to stop being the go-to person. It's okay to take care of yourself - sometimes a serious illness teaches us that. And if people have disappointed you by not stepping up to the plate, then you have to see them for their limitations.
I think the best way is to calmly say, "I'm sorry, I'm not able to do X for you." Don't explain it, don't try to come up with a list of pressures and obligations you have that trump their situation, don't get into a rant about how hurt you are by their failures. If they haven't seen your side of things up to now, they may never - you can't demand that they walk in your shoes or develop empathy. And telling them how ticked off you are can often just sap your strength, which is bad for you.
If you focus on your own backbone and your own principles, if you tell yourself that you have survived cancer for a reason (perhaps that you are meant for better and more positive things in the future), then keep yourself on a positive path! There's a saying that, "When people show you who they are, believe them." The good people will realize that you did a lot for them just by your not doing it all the time, and the selfish/clueless people will work on finding someone else to ask while they pity themselves in the meantime. If you stay positive and strong and not angry/bitter, you will stress yourself out less and attract more positive people.
You can ignore calls/texts, or you can reply SIMPLY "Sorry, no can do. Good luck." If the person persists, keep it so simple - "I thought I answered you that I'm not able to do that." But keep out the explanations, which are defensive in nature and sapping of your strength. Whining/complaining people. "Sorry to hear that. I'm still catching up after having cancer so I sympathize. Hope you find an answer soon. Bye." (Only say that cancer sentence if your cancer recovery is fairly recent.) Now and then, if you are helping someone out who WAS there for you and the whining person comments on it, you can say "You now, Isabelle was so wonderful to me when I had cancer and my husband had to work to keep the benefits going, I desperately want to be there for her now that she is in need. I'm sure you understand. Bye-bye." But the trick is to get the message across succinctly and without accusation. Think of these calls as being 30 second commercials - you only have a brief moment to get your message across. Some people will get it and buy what you're selling, others will be oblivious. That's okay.
Just don't let them all suck the life out of you. People will eventually stop asking you. If you block their numbers, it may make you look vindictive. But you don't have to answer every one either or reply to every message/text.
Move on - there are great people out there just waiting for you!