I wouldn't be too hard on yourself at all. What is your team comp by the way?
I tried to run an All Melee team that was ilvl 415~ avg through a mythic and it was brutal due to some of the mechanics.

Make sure you are upgrading your cloak/doing the assault objectives for the 445 pieces as well as the emissaries when they come up. There's also gotta be an easier way to get a higher ilvl weapon I'm just not sure which route it would be.

Atal is definitely a harder dungeon for mechanics on it. The green goopy guy is also going to be a pain, even the dinosaur and the last boss can be trouble.

Freehold and Waycrest are our best choices.
Then you have Tol Dagor / Motherload / Temple of Sethraliss
Once you get the hang of it Mechagon is a lovely place.
The remaining ranking from best to worst would be : King's Rest/ Underrot - Shrine (kind of tied) / and Siege being the absolute worst last boss.

They also changed Mythic + this season where you get max ilvl upgrades completing a +15, this used to be a +10 so it feels like the curve is much harder in that respect.