OP, I don't know you're game so I can't say this as factual, but my guess would be that your tournament issues dwell from a lack of mental strength/preparation. Once again, this is just an observation from an outsider but typically what I see in people that don't play well in tournaments but are otherwise good golfers is that they never recover from poor holes. The pressure and emotion tends to keep them from playing their normal game. I was playing with a guy that started pretty well, was only 1 over after 6 holes, but on the 7th he took a double bogey. He was so shook up by the one hole that he was never able to get back to his normal game, and went on to shoot a 93. He was trying to hit hero shots and gain all his strokes back at once, which isn't possible. The one hole completely blew his round.
It's much easier said than done, and I'm guilty of it myself at times, but it is important to not get too high or too low in tournament play. I'm not sure if this is the case with you, but with a 42 stroke difference between your rounds I feel like it has to play a role. I don't mean this as an insult, but someone capable of shooting a 78 should not shoot a 120 unless they are playing the tips of a pro course. Inflated tournament scores are normal, there's nothing to hang your head about in that regard. It's just that the difference from your handicap indicates that somewhere along the way you mentally lost focus and couldn't re-gain it. Play for par on every hole and forget the holes before it. You can't change them.