Al Robertson doesn’t believe a marriage always has to end after an affair . The eldest son of Phil and Miss Kay Robertson of “ Duck Dynasty” fame is joining his wife, Lisa Robertson, to confront their painful past in a new Lifetime film premiering Saturday titled “Faith & Forgiveness.” The movie reveals how Lisa became involved in an extramarital relationship 15 years into their union — and how their faith ultimately restored their relationship. “When unfaithfulness happens in a marriage, so many times, that’s the end of it, but it doesn’t have to be,” Robertson told Fox News Digital. “Everything can be worked through.” “I think the reason I was willing to fight for my marriage was that I had made a lot of mistakes on my own,” he admitted. “When I first met Lisa, she was actually a good girl, who had experienced some hard things in her life up until that point, but really was looking for what she called her ‘knight in shining armor,’ someone to take her hand and lead her out of some things.…