How do I learn to TRUST again?

My husband of 14 years betrayed my trust last year when he got an STD from a GRO during one of the "boys' drinking sprees" when he said he got so drunk that he didn't know anymore what he was doing. With so much love in my heart, I learned to forgive him and thought that everything will return to normal. However, this year, he befriended a girl who he said is merely a friend, and sometimes I want to believe him, but more often than not, I always become suspicious especially when I discovered a 2nd cellphone which he tried to hide from me. Should I trust again... knowing fully well that I may be hurt and make a fool of myself? Confronting him didn't work because I didn't have tangible evidences to prove my allegations. I am becoming a paranoid, please help. What should I do?