DON’T deal with this alone
Dionne Warwick nailed it with her 1987 ballad about what friends are for. Don’t be afraid to use them! Call them, meet them for lunch, have them over for movie nights, and let them take care of you. But not forever. Relying on your buddies too much may strain your friendship. One late-night emergency phone call is fine; a week of 3am rants is not. These people aren't trained (or paid) therapists, who, come to think of it would actually be PERFECT to talk to. A little professional advice and some well-thought-out coping mechanisms can do a world of good.
DO get tested
If your partner’s treacherous act was of the sexual nature, immediately get yourself tested for STDs. Even if they claim to have used protection, you can never really be sure -- can you? Play it safe -- your sexual health is of utmost importance; and some of those infections are for life. Even if your relationship isn’t.