Here's what I know...
(1) Guys don't break up with girls over little things.
(2) Once a guy decides he is attracted to you, he won't break up with you over something that only you notice- that one pimple you got on a Tuesday or the fact that you wore a bra that didn't make your boobs look as big as they usually do.
(3) Guys don't pay attention to how many "xo's" you put at the end of your text message and he certainly won't break up with you over that.
(4) Guys read emails very quickly and they don't labor over every word you wrote and what you meant by them, the way girls do. Absent your writing something really awful, he is not going to break-up with you because of your LOL or because you had too many typos.
(5) It is natural to question the last little thing you did with a guy if he ends things with you and you are not sure why. However, to find out what really went wrong, you need to dig deeper and look at the full picture and the patterns of behaviour.
(6) When you are dating someone, it's more important to learn the things that would bother HIM, not obsess about the things that would bother you that don't even register on his "notice" meter. (Example: he won't notice if the smiley face in your text was a full smile or a wink, but he will notice if you need to have a "relationship conversation" by email during work hours, when he is in his office!