If you are listening to a commercially released recording in a perfectly treated room with perfectly flat response monitors set in perfect positions, your ears will receive all areas of the frequency spectrum at the correct level as the mix engineer / mastering people intended.
In real life, your monitor speakers will artificially make some parts of that spectrum louder or quieter than they should be. Your room / speaker positions can make this far, far worse.
Then we have your ears.... it’s entirely possible that years of abuse have taken off the top end of your hearing.
You cannot do much about this last point. The rest? You can address. I treated my little room here by building panels out of 4” acoustic density rock wool which were encased in wooden frames and covered with breathable fabric. I’ve got one on the ceiling, one on each wall closest to my monitors, behind the monitors and then there are massive wall to ceiling efforts to make a vocal wall (for when I record the vox) and the wall opposite. Cost me around £250 all in and made my monitor speakers sound considerably more expensive :).
I am still not great at mixing - it takes a lot of time, effort and experience. But at least now when I make bad decisions in a mix, it’s on me rather than unavoidable due to me not being able to hear it accurately because of the room.
Put it this way. If I started painting portraits, I’d suck at first. Experience would make me better.
Trying to mix in a room that is untreated is like trying to paint a portrait in the dark..... it means you’ll end up with a crap picture and, furthermore, you’ll have little chance of improving because you can’t see / hear what is actually happening so you never learn repeatably what works and what doesn’t.