How do I add a specific topic and specific impression to this sentence? I'll never forget her.

It really depends on the context that you are putting it in, for example, the topic will depend on the preceding and following sentences. However, if you want to change the sentence itself, you can say "I'll never forget her because ... " Thus, the reason and topic will be in the sentence itself. For the impression, one other thing you can do is change the way the sentence is italicized or bolded. That would place the emphasis on certain words and thus change the impression.