From great to...not so great. What am I missing?

So I’ve been playing Halo 5 now almost every day since it came out, and I am finding this game to be beyond frustrating. Not bad, mind you, just frustrating. Warzone is a ton of fun, and Shotty Snipers was the only playlist I felt like I could actually do consistently well in. Just recently, I threw my controller down on the ground so hard that I am still trying to find the battery case (an immature rage behavior I have not displayed since Halo 2). I am by no means a terrible player. I won’t brag about my stats here, but you can check out my service records from across all Halos; you’ll find that I’m good. However, I don’t even feel like I’ve earned the Onyx/Diamond rankings I have. In every Halo, I have done very well across all playlists. I have my good days, and my bad days, but in Halo 5 more days than not feel like bad days. What’s going on here?

I’ve heard the typical, “Just practice, you’ll get better,” and, “It’s a new game with a new feel that you have to get used to.” Both valid points, but I have never struggled to get good at an FPS before in my life. Titanfall, COD, TF2, UT2004, Q3, TacOps, Red Faction, AVP, Half-Life, Duke Nukem, Doom, you name it. I’ve played so many and never had this much of an issue adjusting. Is it really me? I’m certainly open to that possibility, but if I am honest, I sincerely doubt it. Something with this game is off. I know I’m not the only one that feels this way.

To make matters even more complicated, we reach an impasse when I see people doing demonstrably better than me at this game. Could this be the biggest piece of evidence that the issue is me? Once again, I sincerely doubt it because I have my games where I mop the floor with some of these guys. I just don’t get it. My ultimate conspiracy theory is that during Halo 4, all of the tryhards left for other competitive games, leaving me with a lot of casuals to reign over, now the tryhards are all back and I have found that I never got any better by constantly playing against people of vastly lower skill level. Or maybe the game is actually flawed.

What I believe to be wrong with the game is the same stuff you’ve all heard: The aiming is terrible, spawns are questionable at best, maps are claustrophobic and have bad sight lines, etc. It all combines in a way that makes it seem like practicing isn’t going to get me anywhere any time soon. I wish to state that this game does not suck. It is very well made and as a Halo fanboy, 343 did not disappoint me.

So my ultimate question is, if it’s not the game, what am I missing?

TL:DR
This is a good game, but as a seasoned FPS/Halo player, something feels very off. Validate my frustration by agreeing with me!