Do your best to retrace your steps, like what you were doing, how you were feeling, etc. It helps a lot!
Don't beat yourself up about dude, pick yourself up. dust yourself off, and say now this time I am doubling that.
Well done for reaching abstaining for 42 days- think about the power you tuned into to reach 42 days. Its a long journey full of bumps and humps- keep plodding along- You can do this! i'm in the same boat brother! I have relapse so many times, but I believe I can still do it. Come one bro you got this!
Keep it up, it was just 1 out of 42. How was that compared to the past? I'm guessing that you've made progress. Stand up, and keep going, that's the price we have to pay for involving with porn. Stay strong!