I would say that it depends on the individual whether or not it's worth it. I can't speak for anyone here because we all have similar yet different experiences.

I'll give an example of my past for example. These last 3 years I've been in community college, I've met different large groups of people each year. And each year, I am part of the group or in some cases, I am like the mom of the group. I always gave neutral advice to people (with all possibilities, not just what they want to hear.) I've been there for emotional support, I've been there to listen to people vent, I've even given money to people who had no food to eat weekly. Sometimes I never saw them buy any food with it,so lord knows where the money went. I would do them little favors here and there. I was essentially the glue that tried to keep the group together. But majority of the times I found out these people I treated nicely and gave my 100% talked **** about me behind my back. And the saddest thing is they never bothered to get to know me. They never bothered to sit down with me,ask me questions, and get to know who I really am. They just went off of what they heard from other people. Despite it being college, people still acted like immature highschoolers. It doesn't help that I have BPD but they don't see me as a broken hurt person. They chalk it up to whatever negative stigma they have of mental illness (which may I add,they lacked any compassion for anything.) It was a group comprised of two faced people,thieves that stole what they could afford,stole from friends,people who cheated on their significant others. But that's another story for another time.

TL;DR I treated people nicely,and they stepped on me,as you've also said you've been through being stepped on;living through toxicity

It's up to us to determine whether or not it's worth it. Should we continue being nice? Should we stop being nice all together, slow down our roll,etc? I'd say measure the pros and cons of you being nice and giving 100% but not getting the same back. That's one way you can use to see if it's worth it or not in the long run. Personally for me,a lot of people say it's not worth it, but I've always told people. If I can't make myself happy, the very least I can do in the world is make everyone else I know or meet happy. That's enough for me. Even though in the end, sometimes it's a cycle of be nice,get backstabbed,get hurt,and such. I will still move on, find other people, and help be there for them for anything. I never had anyone for me when I was younger,so I want to be there for others. Just try not to let this be the reason why you start hating humanity or losing hope in people,because there are good people out there. You'll meet them one day and have a worthy friendship/relationship. I've also met people who enjoy the feeling of being nice even if it doesn't pay off,because they go to sleep feeling good about themselves. That can be another way to look at it.

Sorry my advice isn't really the best. I hope you find peace in the end OP. Remember. There's always rain before a rainbow.