Translation:Leaving is so difficult, staying is just beautiful.
This sounds right. I think it's a forgotten alternative that - because of the uniqueness of your translation - has never come up before, you can report it.
I think this sentence is from the gerund-section (I'm reviewing, so can't see where it's from). Gerund means that you can use the form verb stem + ing ('leaving'/'staying') as nouns. Is also correct here to use 'to leave' and 'to stay'.
When you use the full verb form (to leave/to stay), is implied that the action hasn't happened yet. Use of the gerund (leaving/staying) implies that it happened/is still happening.
Did you read the notes/tips on this section?