Classical musician here. I'd start off with a good guide to the history of music. That'll give you an understanding of why what we call classical music has so

Classical musician here. I'd start off with a good guide to the history of music. That'll give you an understanding of why what we call classical music has so much range in it. The context helps immensely. After that, there's an element of accepting that the longer forms of music are going to take a while to listen to. Remember they were meant to be performed by musicians, with an audience listening from start to finish. The bulk of what you'll probably think of as classical music is essentially concert stuff, rather than 3 minute standalone pieces. That being said, there's huge range in the canon. From short practice pieces and tone poems which stand alone to huge many hour opera pieces, and everything in between. I'd start with The Story of Music by Howard Goodall, and How to Listen to Great Music: A Guide to Its History, Culture, and Heart by Robert Greenberg. After that, pick a period and start listening. Feel free to ping me any time for more information. EDIT: It's also worth noting that there's huge range in any particular piece depending on who's conducting it. For example, if you take a commonly performed piece like the Overture to Le Nozze Di Figaro (The Marriage of Figaro), you can get everything from Teodor Currentzis to Georg Solti to Karl Bohm. Three different styles of playing exactly the same thing. So it's also, in time, worth looking into how music is arranged and performed and how that affects both what you hear now, and how people composed at the time.