I can't figure out the middle of my book! I've figured out the beginning and the end, but I don't know what should happen between my characters so that everything ends well and happily? Could someone help me with this question?
What happens in the middle is the worst possible thing, walking your characters directly into catastrophe, into horrible losses, so they think they will certainly fail, and let themselves down, and the people that are depending on them to succeed down, and possibly even lose their lives.
In stories, the essence of a "hero" is that they never quit.
Die Hard is the perfect example, John McClane (actor Bruce Willis) in Die Hard. McClane would literally rather die than fail, and risks his life again and again. He gets cut, burned, bruised and torn, he fails time after time, and every time he struggles to get back and try again.
That is what your hero has to do. Power through failures, power through heartbreak, power through their own screw ups and misunderstandings and emotional trauma, to finally bet it all on a million to 1 longshot -- and emerge victorious.
If you want to be a successful author, you need to learn to be cruel to your heroes. No matter how much you love them. If you want readers to love them, you have to give them a reason to love them, and that reason is their courage and commitment in the face of long odds. Despite their doubt. Even despite their personal conviction they will probably fail, but they have to try.
As Stephen King says, in Act I you make a lovable character, and then you shove them into the blender. Hurt them.
The parts you've done, the beginning and the end, are the easy parts, because you can write as if you love your characters.
The middle is the hard part, you have to kick them in the face, stomp on them, cut them, burn them, and slam them against the wall. Make them terrified. Make them cry. And finally, make them angry, so angry they have to fight back. And fail. And fail. And fail again, until they finally know enough to take a wild chance and maybe, maybe, get it right.
Then you can write the final chapter, your victorious hero regaining what they've lost, or starting a peaceful life in their new "normal".
It's not clear what kind of book you're writing, so it's hard to be specific in the answer.
As Amadeus pointed out, the middle is the difficult part where the characters and the challenges go head-on. If you've made the reader identify with the characters, do something unexpected. The threat of a negative outcome should become apparent to the characters, and some may meet a dreadful fate.
Stephen King, George R.R. Martin, and others have been brutal to their main characters. It's easy to fall in love with your creations. Don't. Kill them, maim them, change them. The end of the story should reflect the outcome of the characters' experiences with the challenge.
Best of luck!