I used to be a two-spacer for the longest time because that's what my high school typing classes taught me. Guess what's made me switch to one space? Twitter. Every character in a tweet is very precious. I couldn't spare that extra space. For consistency's sake, I've carried over one-spacing to all my other writing.
And the abbreviation problem? Simple. I don't use periods for them. I've been doing that long before Twitter came around. I guess I've always found periods after abbreviations redundant. "No, Mr Bond, I expect you to die." Everyone knows Mr is short for Mister there. So I don't bother with the period.