It's one more step and you have taken it bravely. Well done. I read your message a while ago, but I had a different type of relationship with my mother so I didn't know how I'd handle your situation. I am glad you have found a way through it and hope you start feeling good about the split being 'official' now. From what I've read about your ah, you have every reason to feel good about this.
I know you have been unsure about how much to tell your parents. If your parents start asking you questions, do you think you will keep to the statement in your letter about your ah moving out last Friday?
Are they likely to speak to your ah about the break up? will he say he moved out months ago? if your parents end up with two different versions of events will this cause problems for you? I hope I am not sounding negative, I just want you to have whatever support you can get from them - are they the sort of people who will expect the truth from you in order to give you this? I really hope their support is not so conditional.