Downshifting a Kenworth T700

I'm currently OTR with a trainer, but he's not too helpful so I'll ask here.
I'm used to driving a 10 speed Freightliner from CDL school and now I'm with a
company and I'm in a Kenworth T700 8 speed.

When I approach an off ramp I need to be 10mph below whats posted.
I could be in 8th gear going 50 mph and then I need to slow down to
25mph. I'm slowing down in 8th gear but when the truck starts to sound
super rough and I attempt to shift into 7th and it won't go in the hole. I tried
6th too and still grinding and won't go in.

Am I doing this wrong or could someone walk me through this? The trainer
isn't very helpful. In fact I'm in the jump seat while I'm writing this.

I appreciate the help! Thanks in advance!

 

Yeah im with Stevens. When I ask certain questions, his response is, "its up to me". I dont know much and I'm almost thinking to switch trainers. He drove some of my miles too! I don't want to be a snitch but its taking away from my experience. I think these "trainers" use us for extra money and careless if we learn or drive. Only reason I'm
with Stevens to pay off my loan and pay for my Haz Mat. I'm
gonna call somebody tomorrow and talk. I've only backed 2 times into a regular parking spot. I havent docked at all and I sucked at it at CDL school but I passed the test. Ugh I dunno what to do!

i'm in OR1 and I think I'm on my 4-5th week and I havent drove or backed much. I've played on Facebook longer than I drove!

 

My trainer wants me to slow down to ramp speed before I start shifting so I think now im confused. Ugh! Everyone shouldnt be a trainer, especially leased drivers for a company. My oh my what did I get myself into? I thought Stevens had a great training
program!

My trainer said dont snitch on him but
its not fair to me. I dont want to become a poor driver or
get made fun of because I can't drive.