I personally agree that writing a letter or a message to her could be beneficial. You would be able to take your time and express to her what you felt you were unable to express to her before, without feeling pressured. Let her know why you acted the way you did in the relationship, and how much she meant/still means to you. I also agree that it's best you don't expect a response in order to spare yourself from possible disappointment. You would have at least tried, and getting it off your chest may give you a little peace of mind.