Is "treat others how you would like being treated" a worthless teaching?

I've been doing that for so long. My parents raised me to be a well mannered child. I was taught that being decent gets you kindness in return. I'm about to give up their teachings.
As a kid, I got bullied a lot just for being shy and "too sweet" (that's what the bullies said). I got scammed by an ex boyfriend for money. I asked why and his reply "why not, your rare but easy to fool."
My last straw:
2 years ago I helped a female beggar that told me a very sad story about her life and being abandoned by her family. I gave her $300. Then an hour later, not only do I see that same woman in other clothes but overheard her talking bad about me with her friend about how I was one of the most naive person ever. Then she added the words "I'm telling you she's a rare breed"
I had it with people. Nowadays I've abandoned several old fashioned teachings, became cheap and put myself first more times. Is being a decent person and treats others well worthless in this time?
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+1 y
And here I am wondering what on earth I ever did to get poor treatment. When I was all giving, they all took advantage, scammed or lied to me.
When I started becoming meaner and tough, people back off.
Is "treat others how you would like being treated" a worthless teaching?
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