I'm sincerely happy that Koichi's incurable disease actually has been cured.
I think Koichi hates me from the bottom of his heart.
There are a lot of nice people in my new department and I feel tremendously relieved.
The heart bottom is the bottom of one's heart. Do you want to learn kanji? Do you want to learn it from 心底? Okay, fine, you can move onto the reading then.
This word uses the on'yomi reading for the first kanji and the kun'yomi reading for the second kanji. You've learned them both already, so you should be able to read this too!