He puts it down to being under less pressure.
"I think I've always had confidence," Magnussen told BT newspaper. "The difference is that I'm always been much more pressured.
"I was super-pressured at McLaren and it was the same at Renault because the car was so bad. Now I'm in a very good place in my career with a team that supports me 100 per cent," he added.
The 25-year-old thinks stability in his F1 career is also helping.
"It's actually the first time in my entire career - even outside of formula one - that I've been at the same team for two consecutive years. And it's only now that it really occurs to me how important that is."
Magnussen has therefore staked a strong claim on a Haas seat for 2019.
"There is not much to say about my contract," he said. "They have an option on me, and I hope they use it."