The horror! Memories of high school typing classes! :)
Just took a look at one of my posts and I'm seeing period-space-space at the end of a sentance. I'm pretty good about that as it's one of the few things I still do from my classes. (I have this weird five finger typing method. My teacher gave me a B for it even though I had fairly good accuracy compared to the other classmates.)
Since you're using the HTML button, I have a feeling that you're using the rich text editor (otherwise know as TinyMCE) and it wouldn't surprise me if TinyMCE was stripping out the extra space and mucking up the lines. (Heck, it puts extra code in there, why not take something out as well)
My advice is to change over to the normal WordPress editor. You can do this at Dashboard -> Users -> Your Profile -> Uncheck the box at the bottom of the page and click on Save Changes.
We had a thread a couple days ago on space lines being deleted. I think the outcome of it was to do
<br /> lines instead if they were still being removed but changing your editor would be my first step.
Hope this helps,