Haven't found mistake in sentences

Would you kindly tell me if there are any mistakes in the following sentences?

->I haven't seen her since [6 months] before I met John.
->That's everything I don't want in life.
->You should tell yourself that to make yourself sleep better at night.
->Just thinking about the pain she is going through makes me hurt.
->You can't know too early in advance what the weather will be.
->Her sense of smell isn't/hasn't developed.

Thanks