Jumping a stubborn horse...

I've been riding Maestro for quit some time in my lessons now. He has days when he's either really forward going and jumps really well or then he has extremely stubborn days.
For example today, the beginning of the lesson went well. He was nice and forward going, but at the end he just turned into a stubborn git. Whenever I try to drive him forward he just bucks, which means we lose momentum. Doesn't matter if I use my seat and legs, whip or spurs,he just kicks out. I was the only one to ride him today.
What can I do to make him go forward?

On a bright note, I jumped my first triple bar and hog's back today and they went well. And the triple bar was about 1m high

Idk what to tell ya you just got to try to stay calm and happy at all times cause if you get stubborn and frustrated also your horse will just feed off of that.... maybee try to do grown work making him trust you and wanting to please you, make it fun not like a job, so his more willing maybe.
Good luck my horse is pretty stubborn also and what seemed to help me the most was just grown and trust work and now I have a permanent forward seat lol ;P but I love him loads :) have fun

Unfortunately he is not mine and I only get to ride him during my lessons once a week.

How many jumping lessons does this horse do a week? Maybe he's just getting up there in age and can't keep up with the volume anymore?

1 or 2 and not too much jumping during the lesson. He clearly enjoys it when he's forward going though.

I have only ridden for seven years, but I have a nack for stubborn horses. I am the one to ride them at the barn. Be patient with him. Try not to spur and whip to much, that may make him more stubborn. When you feel him becoming stubborn and not wanting to move foward let the reins a tad longer than you would normally, but make sure you have control. Loosen your grip as well. Also, if you can make him go in circles in every corner. That is good for both hyper horses and stubborn unmoving horses. I know it may be tricky in a lesson. If he stops before a jump or acts up going over one, make a harsh clicking noise and urge him, squeeze him as hard as you can, make sure he knows you're the boss. ALso, think happy, moving forward thoughts... sounds crazy... it works though! Hope these tips help. Good Luck!!!

It could be one of two things, in my opinion.

1) He knows when the lesson is starting to end and he doesn't want to keep working. Or something similar. What time are your lessons? Before dinner time or another meal maybe? I've known several horses that either need to be ridden well before a mealtime or after they've eaten.

2) He's in pain and is becoming sore. I know you said he doesn't jump more than once or twice a week. How high is he jumping? I know you said that you jumped about 1m today, does he usually jump that high or is it much lower? Has he had any conditioning work or flatwork to help build up muscle?

The lesson is at 20:00 and he get's food after. I really do think he knows that the lesson is ending.
During the lesson we don't even jump all the time. First it's about 15-20 mins of flat work and then when jumping we have to wait for others to jump (there's 8 of us) so it's not like it's constant work throughout the lesson.
1m is the highest we usually jump and not during every lesson. The heights vary from a small crosspole to 1m. His muscling is good. He's very well conditioned.
He also has hates other geldings and tends to attack/buck at them whenever they are too close. And his personality is very quirky.

He was much much better today. We practiced really sharp turns which he is really good at and then jumped a 1m course and he was much more forward going. Had 3 stops. 2 were rider error and one was a dirty stop.
He just has days when he's really fun to ride and days when he's a complete cow.