So basically your ex- is twenty, you're fifteen, and you're upset that you're no longer able to date her? And this followed one or both of you ignoring your respective parents' instructions that you were not to see each other? And she's on probation for a few years, suggesting she has a felony conviction of some sort?

You can send whatever you want to the probation officer. You have no standing to object to her terms of probation, or to bring a motion for modification of those terms, so the net result may be that the probation officer puts your correspondence right into the proverbial circular file. (Your request could have the opposite effect from what you intend - it's possible that the correspondence will convince the probation officer that the restriction was and is a good idea.) What happens is up to the probation officer, but I expect that if the probation officer found sufficient cause for concern to impose this condition the probation officer will see little reason to lift it early.