I think it would be a good thing to do when the time is right... along with a detailed explanation of what depression is and what causes it. But it isn't exactly something you just want to blurt out for no apparent reason. It's best if you can work it into a relevant conversation. Also, try not to make it sound like an excuse for treating her badly... I think that explaining it as the reason you've been treating her as you have, along with an apology is a good way to go. That's what I've done in the past! But I don't think it's a very good idea if it sounds like an excuse that you're using to justify yourself and let yourself off the hook for your actions.

Dan