Is acting very hard? Can you teach yourself?

No - you can't teach yourself. But in most countries/cities/towns they have community theatres and many of them offer classes at very reasonable cost.
Or you could join one of them and help out backstage or selling tickets, refreshments, doing the costumes or make-up - anything they need - and learn from watching the actors rehearsing and performing.
After a while, you could try out for a small role in one of their productions.
If that goes well, you could work up to bigger and better roles.
It's great fun - give it a try!

Acting is not something you can learn alone or practice alone. Acting is about listening and reacting to the other people in a play, scene or movie.

You can do it as a hobby by joining a community theater but you will have to practice it with others.