Mahler is an acquired taste. Start with the 5th symphony. If that’s too long for you, just listen to the scherzo. It’s still long, but worth every second.
Mahler might not strike you as modernist at all. After all, wasn’t he just a very late Romantic? And wasn’t everything that followed modernist? Breaking the tradition Mahler was a part of?
There is truth in this, although Mahler pushed the tradition further, influencing what became known as the Second Viennese School. But Mahler’s relation to tradition, the tradition he was a masterful conductor of, was already a broken one. Being the director of the Vienna Court Opera, nobody could have been more inside the tradition than Mahler. But Mahler, as always, was also an outsider, an ironic commentator on this very tradition. Mahler didn’t just work within the tradition, he tweaked it and developed it further.
This is his true contribution: work within a genre, a style, fully aware of the past greatness achieved within it. Dare to. Accept the tradition with its standards and measures against which your work will be judged. There is a sense in which this takes more courage than simply breaking with all of it.