I have no idea how to stop feeling like this

Before i start describing my problem, i want you guys to know that i'm not a native speaker, so please excuse my english since it's not my mother tongue.
*mod edit* i have this problem where i can't see people other than selfish creatures, including myself. I "realized" that we do everything in order to please ourselves. For example, if person A falls on the ground, person B will go to help her not out of concern, but because person B wants to feel good about themself. If person B misses person A, they don't actually miss the person but they're scared that they will never see them again and thus not being able to share happy moments with them. By thinking like that, i stopped believing true love exist, or even platonic love. When I watch a show or a movie, i can't enjoy it since i always find a way to think the characters are selfish people, same things when i go out of my house. And i can't help but to think like that. What I hate is that even I am also selfish, since by writing here i want to stop worrying about those things and be happy. I know it's contradictory, but i can't help but see happiness as a sin. Three years ago i was bullied and my friends gave up on me just like that, i think that's about then that i started thinking like that. But what's really hard is that i can't stop thinking like that. I think about that days and nights, and i can't trust anyone. I'm very tired. And i feel guilty for helping people, because maybe i'm not completely sincere by doing so. Thinking like that makes me very desperate... and lonely. I've been depressed for over 4 years and i think i hit the very bottom now. When people tell me "i love you", i can't be happy. I became scared of the word "I". I lost interest in everything, i feel dead. Unreal. Disconnected from reality. To me, being alive is strange. And i started to think that life is not worth living it. I feel so lonely. I'm scared i will be like this for the rest of my life. My head hurts, but i can't stop overthinking this stuff. I'm sorry if i hurt anyone's feeling here, and sorry if i posted this in the wrong category. Has anyone felt like that before ? I don't know how to be normal anymore...

I am sorry to read that, my friend. I can't help you, since I don't know anything related to that and I could end up giving wrong advices, but I'm sure someone else here might be able to help
Just stay strong.