They look very good for the most part, but there are slight differences between the sentences:
This relation won't work. --> This is not specific to a relationship between people and sounds awkward if you are, in fact, talking about a relationship.
This relationship won't work. --> so this sounds more natural.
I don't intend to stay here.
I have no intention of staying here. --> These sentences are completely interchangeable.
It's not like anything I have ever seen before. --> You could also say "It's unlike anything I've ever seen before."
It's like nothing I have ever seen before. --> But both of these sentences work correctly :)
There is something I didn't mention to you. --> This sounds awkward in conversation. I would drop the "to you" because it is implied in conversation.
There is something I didn't tell you about. --> This is perfectly correct - Bravo!
Then I have learned something I will never forget.
Then I have learned something I would never forget. --> the "would" in the sentence seems out of place to me. "Would" in this context does not adequately replace "will" like how it would if I were to say "I would/will never do that" or "I would/will never forget that." I would change this sentence to "Then I have learned something unforgettable"
I hope this helps!