In such case it's okay to not write the thing.

Or to write it crudely- with errors and naivete, bursting with emotion and letting whatever it is inside you to flow on paper, like kids do. It's okay too.

Or to painstakingly work on the letter, stumbling and rewriting and reading, and then rewriting again and again until what you read matches how you feel.

Most people are very forgiving of poor writing skills when facing something sincere. Instead of suffering through some shallow word soup that could have been a mediocre press release, a reader will see a soul behind the stream ot utf-8