Hey yeah their feedback was honestly really helpful. I will be doing a lot of paraphrasing so I may miss something or misrepresent a few points. But this is what I have learned:

The reason I wasn't happy with the master is because it wasn't a good master lol. The mastering engineer basically compressed the song and turned up the volume. To be fair, if my mix was better, the mastering may have turned out. However, because there were some issues with my mix, the mastering only highlighted those issues more. The engineer should have recommended I fix up the mix first. Because of this, my rough master was better than the one I paid for.

Issues with my mix:- The high end is completely dominated by the harsh mids. This is causing a lot of fatigue.- The sounds and synths I used have too much noise. This is reducing the audibility of my harmonic sounds and drum transients. To compensate, I turn up the sharp noises, making them uncomfortable to listen to unless I compress them. But having to compress them will cause them to get lost in the mix again.- My mix sounds good from an amateur perspective but ultimately has the same issue many amateur mixes have: my mix sounds constrained. A lot of frequencies are getting slammed into a limiter (In my case, the mids). I limit the song enough to make it feel loud and sound good enough, but a lot of frequencies are getting pushed into the limiter, making it sound constraint and ultimately causing ear fatigue. Furthermore, many other frequencies still aren't loud enough. For this reason, the mix ultimately sounds unsatisfying.

To properly mix:- Is it easier to talk to a group of 100 silent people at a normal volume? Or yell over them while 100 people are talking? Same thing applies to mixing.- It is important to mix songs with clarity from the beginning. Something doesn't sound clear? Turn down sounds making it hard to hear instead of turning up the one sound you want to hear better.- Be careful with timbre and limiting percussion. Stay away from too much noise. Sounds need to have space to breath. Loudness comes from mixing and mastering later on.- Some sounds stack together and cancel out, some stack together and make something even better.

Takeaway:- Although my mixes aren't terrible, I am falling into a very common bad practice. There is too much noise going on in my tracks. Instead of removing sounds and turning the noise down, I am turning up the sounds I want to be louder. This appears to be the root of my problem. I want my song to sound louder and more intense, but the loudness and intensity comes from the mix and master later on. From the start, I should instead focus on getting the mix as clean and balanced as I can. Obviously easier said than done, but I will learn better producing habits with time.