If I didn't see you tomorrow

[Stlouis]

"If I didn't see you tomorrow, I would kill myself."

Is this a grammatically correct sentence?

 

[Raymott]

It would be far more common as "If I don't see you tomorrow, I will kill myself."
I can't think of any context where your version is better.

 

[teechar]

"If I didn't see you tomorrow, I would kill myself."

Is this a grammatically correct sentence?

Yes, it's grammatical and possible. For example, consider this conversation between a couple who are besotted with each other:

A: Would it be so bad if you didn't see me tomorrow? I mean, it wouldn't be the end of the world, would it?

B: If I didn't see you tomorrow, I would kill myself.

 

[Tdol]

It would require some additional context to make it work, like Teechar's example. Being grammatically correct does not automatically make something a good sentence.

https://en.wikipedia.org/wiki/Colorless_green_ideas_sleep_furiously

If you really want your head to explode on the difference between grammar and meaning, try this gem:
https://en.wikipedia.org/wiki/Buffalo_buffalo_Buffalo_buffalo_buffalo_buffalo_Buffalo_buffalo