I am sorry about this - and the result.
I'm not going to waste your time debating the rights and wrongs of how you got the information - that is done now.
Without knowing either you or him, I feel I'm on thin ice dishing out hard-and-fast advice, but i would ask you to consider this -
Your BF has been confronted and tried to hide his actions, stupid and pointless, but still human nature - that's not an excuse, it's an explanation.
Give yourself time to think about this - alone, and decide where you want to go from here.
Ignore him totally - the very least he deserves is to seriously sweat for a few days - and think about whether or not you, that's not you as a couple, but you as an individual - can climb over this, or not.
No relationship is over until you have gone through this process - you need to assess whether something that has been done, and gone, has hurt you beyond repair, and that is something only you will know.
The point is, you won't know, today, or tomorrow, or for a while yet. You need the initial shock and pain to subside before you get down to what you have, and are losing, and whether in the long run you cannot get past this.
The important thing is, if he is gone today, or gone in a month, he is just as 'gone', so don't rush into anything.
I am sure I join with my fellow AB'ers when I say that our cllective thoughts and sympathy are here for you at any time - so feel free to talk it over here if it helps.
I am so sorry.
A x