Why haven't you confronted him? Get it done, or nothing will ever stop the "doubts" you have, they will just become more and more, and you will be suspicious of everything he does. Be a Woman, and take the bull by the horns, and talk it out with HIM. It had nothing to do with "her" - he "blew it" - put this anger in the wright place. And if you don't get a satisfactory answer, if it just doesn't make sense, its a lie! Remember that, and no friendship,love,or relationship will last, when it is built on a lie! Better to let the cards fall where they may NOW then latter.It doesn't get better from now on, especially since you know. and it doesn't matter that it was a year ago - that just makes it worse, doesn't it - that he "played" you all this time. Forget that he is a no-nonsense kind of guy - SO ARE YOU! - set him straight, for once and For All. Good luck girlfriend - i know you can do it. Now DO IT!!!
From a mother who would have taught you better and supported you now.