I can understand you not wanting to give out too much info but it's hard to give some good advice with what you've written.
For example, is this a first relationship for you both? If it is, then it is common for first loves not to last, especially when you're young.
You say "...started to get comfortable in the relationship." Generally speaking, there is nothing wrong with getting comfortable in a relationship. It's a good sign that you both love and trust each other plus you are free to say what's on your mind and be yourself. So, I don't understand, why do you see "getting comfortable" as a problem?
If she felt unappreciated, did she ever express that to you before the breakup? You say you weren't "keep(ing) her interested". If you mean that she was bored and her feelings for you waned, then there isn't anything you can do if her feelings for you have changed.
You could try writing her and letting her know your feelings about things. However, once you've sent that to her, then the ball is in her court. In other words, it will be up to her whether or not she decides to respond.
Along those lines, when you broke up, did she say she needed time away from you or was the breakup more final? If it is the latter, then it's best for you to take the time to mourn and then move on. If it is the former, again the ball is in her court. You could ask her in your letter if she wants to get back together or if she wants to call it quits. If she just wants some space, so you're not left hanging, ask her to give you a time line. Does she want to see how she feels after a two months, for example? If so, then check back with her then. At some point though, if you don't hear anything from her one way or another, then you need to accept the relationship is over.
In conclusion, with what little you've said about the relationship, to me it sounds like moving on is the best thing for you both.