You cannot regain your trust until he proves he deserves it. The bottom line is that he fucked up, so he has to fix it. It would be foolish for you to simply start trusting him again without him making some changes to show you that he merits trust.
Some simple examples are giving you full access to his email, facebook, cell phone, basically any method of communication that he has, for you to verify at your whim whether or not he's keeping secrets.
Gralson: my husband. Auto: my girlfriend.
Zoffee: Auto's husband. Cue: Zoffee's boyfriend. Bookie: Cue's wife.
"Love is not breathlessness, it is not excitement, it is not the promulgation of promises of eternal passion. That is just being "in love" which any of us can convince ourselves we are. Love itself is what is left over when being in love has burned away, and this is both an art and a fortunate accident. " -- Louis de Bernières