How do you build up trust in a relationship?

hi, my partner and I have been together 7 years since we were 16 and we have had ups and downs like any other couple. Were starting to talk marriage and it came out that my bf flirted with another girl about 3 years ago when we split up and slept with her. Weirdly enough I know the girl and I know it only happened the once and it did acutally happen when we were split up, I can remember the time. It doesnt mean it doesnt hurt though and I feel like somehow my vision of him is tainted, he was this great guy who was so in love with me etc and it turns out that he can stray. I think its a case of me taking him for granted and uming that he would never find another girl attractive. Even though it happened ages ago, I only just found out about it,so the fact he's kept it away from me bothers me. Do you think Im silly to try at the relationship? we both have trust issues to some degree but both feel 7 years is a shame to just throw away. Were quite old fashioned in the sense that we both believe in working at relationships until theres nothing more that can be done. Are we fighting a losing batlle? anyone been through anything similar?