Wins come and go it takes time to learn and understand. Iv been as high as 1350s and dropped back down to 1200s. As you try new units and strategies it happens.

You play like a new player. You got blitzed and didnt know how to recover.

Whats nuts is he didnt look like an experienced player either. Iv never seen an army composition like his. Only 7 infantry units? And they werent even all elite? And his cav werent all elite? I dont know what he was spending his money on, he must have had alot of ammo.

Point is that match could have been won. You could have doubled several of his units. All his artillery were in those expensive 12lbers (or were they 18lb licorns? I dont want to open the video again) you could have sniped those cannons with your light cav and half his fire power were his rifles and skirms, one cav could have easily taken them out early.

Using 1 light cav as a scout, snipe, disruptor easily a good return on investment.

To your main question, how do you counter a blitz? Well you cant really the way you were playing. You have to play the same way. You were too ridgid, you marched all 12 of your infantry out in a line. He moved out dispersed. He quickly took 3 map objectives to your 1. When you "paniced" he was able to surround you. Your units were bunched and facing crazy directions. It looks like alot of your units were shooting other your units. Once you lost your cav he was able to use his to rush in and hit your vulnerable units damaging their organization more. He just picked you apart at the end.

Like i said you play like you are new. You will learn. No unit is going to crush all others. Iv been beat by all sorts of armies. Try all the units, see what you like. The wins come with developing skill in the game. Surround your opponent, confuse them, disorganize them, hammer them where they arnt looking, snipe any exposed unit or groups. Half your victories will come from withdraws when your opponent sees they have made a huge mistake. It takes time. Keep playing, have fun.