After being together for several years, you find out that your significant other has cheated on you with four or five other people. These aren't one night stands either, but, 'relationships' where the shortest one lasted for a month.
After being together for several years, you find out that a few years ago, your significant other hit someone while driving drunk. The person survived, but was injured and will never be back to 100%. Your SO was never caught and hasn't had a drop of alcohol since.