Other Kryptonians seemed to perfect this power rather quickly but it took Clark all ten seasons- was there ever an explanation for why he struggled so much with flying?
Because he was burdened with worry and self-doubt, figuratively weighing him down.
Clark has been able to fly since he was 14, but cannot consciously use it. This is apparently due to his own psychological barriers.
To a large degree this was the overarching message of the entire show!
Notice how meeting real family (Kara) got him close [7x15 "Veritas"], and being a super-confident madman ("Kal-El" under Jor-El's control) took away the problem too [4x01 "Crusade"] … but as Clark under normal circumstances, it took getting comfortable with Lois and seeing fragments of a future in which he completely trusted her, to really begin the journey towards control over his flying ability [10x04 "Homecoming"].
Sadly, the finale had Jor-El just magically turning the powers on completely, rather than showing us some moment in which Clark finally realised he was "ready to go". Oh well.