How do I stop getting so fixated on people?

I meet people online and sometimes I just get so caught up in them that I abandon all my responsibilities and such just to talk to them, and I get so upset when they don’t respond back or like me back. One person who I wasn’t really super interested in in the first place was very nice and said he didn’t want a relationship, and honestly I didn’t either. But I got so upset over it I attempted suicide because I just took everything he said to mean I am worthless. That is obviously not what he said.

I end up texting a lot or calling if they don’t respond back after a while and I just get so worried that I did something.

It’s like everything in my life gets put on hold and all I care about is this person and their happiness and acceptance and all I want is for them to care about me. I get so obsessive where all I want is to make this person happy that I don’t care if I lose sleep or anything I just want someone else to be happy.
And when they aren’t in to me it’s like the end of the world and it just ruins my whole day.

I don’t eat, sleep well, or do anything I enjoy because this person rejected me. I just give everything I have in these stupid little flings and it just leaves me hurt all the time.

But the happiness I feel when I’m talking to someone is really nice and I keep trying to chase that feeling too.

It’s unhealthy. I don’t know how to stop.

All these feelings are just so intense and sometimes I get convinced that my life is nothing without them and that they are the only one for me.