Recently I was doing exercises from my textbook. There was a question that asked me to write a letter. I was thinking about the beginning of the letter, but I can't determine which sentence was correct or the best. Here are three sentences I thought: 1. It's been a long time since I wrote to you. 2. It's been a long time not to write to you. 3. It's been a long time not writing to you.
If all of them are incorrect, please help give me a correct one. And the main question is I don't know the difference between 2 and 3, that is 'to write' and 'writing', as I know they can interchange in most cases. Thanks!