。
I feel more at ease when I'm the one driving.
Literal
I [topic-は] driving-was [direction-方] [subject-が] spirit [subject-が] at-ease [explanatory-なんだ].
~していた方が ('it's better when I was doing') uses 方 for comparison. 気が楽 (きがらく, 'feel at ease, relaxed') — 気 + 楽. なんだ adds explanatory nuance — 'the thing is, I feel more comfortable driving.' An interesting preference statement.