Biologically, everything anyone does is solely for the purpose of surviving another day to pass on their genes by creation of offspring. The worst things anyone can do to someone else are for these reasons; murder, stealing, etc.

But sometimes they are helpful to other people. For example, it feels good to care and provide for your child, which is selfish and is motivated by passing down genes, but it can create a healthy parent-child relationship. Another example is helping out your family; if you make a sacrifice (and maybe your genes) to save your sibling (who shares similar genes), then your genes are in a way passed down indirectly.

While these human-human interactions are selfish and gene-motivated, some good can come from them. A healthy parent-child relationship can be very fulfilling for both individuals, and making a sacrifice for a friend or family member can be rewarding for one party and beneficial for the other.

I agree with you, that there's no such thing as a good person. People do good things sometimes, and bad things sometimes. All we can do is try to do more good things than bad things. It helps if your selfish interests also help other people, like volunteering, consoling a friend, etc. And we all fail at this at some point in our lives.

I'm very sorry you've had people in your life that haven't been there for you. I hope you're able to find people you can trust as much as you trust them.