Welcome back! :)
First off, if your cycles are only 21 day's, I would classify that as an LP deficiency. When are you Ovulating? Do you chart your temp, or watch your CM change? I would get a hold of your OB (if you haven't already) and get in to see him/her, I've never heard of anyone having a 21 day cycle, that just isn't long enough. I would get that checked out first before anything :)
Secondly, so sorry for your loss, I know it was a few years ago, but you never forget a loss :( We lost one (my first and only ever pregnancy) back in 2004, the anniversary of loosing that baby is actually tomorrow :(
But I wish you luck! Are you taking anything or doing anything "different' now that you're trying?
I am sorry for your loss too :( It is a tough thing to go through for sure.