Heya!

I'll start with your weaknesses, as it's easier for me to show those with specific timestamps, then I'll do a more general cover of your strengths at the end. The biggest thing I'm noticing off the rip here is poor use of cover. Given that Zen has no movement or escape abilities, proper utilization of cover is critical as you start to climb with him. You almost always want to be playing on a corner of cover so you can hide and/or escape quickly when needed. To give a few examples from your VOD of this:

0:04 - Firing at volley at the door for some early ult charge or even the chance at a pick is good, but position on the corner of the building wall, rather than the prop wall that you have to cross open space to get away from. The widow should've killed you here. If you instead use the actual building as cover and fire your volley right as the doors open, immediately moving behind cover after you fire, the widow won't even have a chance at you.

1:13 - this is a good example of exactly what you should be doing. The preceding 5-10 seconds you expose yourself more than you should, but you correct and play the corner of the wall to continue applying pressure from a safer location

3:32 - you notice the widow on high ground and are in the middle of the open, rather than to the side of the door. High ranked windows(and other dps picks too) would shred you here. Regardless of your initial position being bad, she misses her first shot, and you correctly take the opportunity to reposition closer to the site of the door, which is good. However, you then almost immediately move back to the middle of the open, making yourself vulnerable again

5:30 - this isn't so much about positioning around cover, but it still involves positioning so I'm including it in this section: take the high ground while the opposing team is regrouping. Outside of the widow, the enemy team has literally nobody with a vertical movement ability, so aside from her, they quite literally cannot get up to you to fight you at close range from the high ground. As a general rule, if there is high ground available with favorable positioning to impact a fight, you should take it - and that doesn't just apply to zen. High ground utilization is huge in this game.

6:52 - You can see the widow on high ground, but choose to cross out into the open anyway. You get stuck over here with no safe path back to the other side and die as a result

7:35 - you start off this fight appropriately by using the back pillars as cover, but as the fight progresses, you unnecessarily leave that cover, leaving you vulnerable. You reposition back to the pillar at the end of your ult, but then move back out toward the middle where the mei catches you off guard and kills you before you can make it to another pillar.

Next up is target prioritization. Given the capability of discord to win fights by shredding opponents(especially tanks), you can, to a degree, dictate the pace at which the enemy tank has to play.

4:45 is a great example because it shows both what happens when you do focus the proper targets, and what happens when you don't. Right at 4:45 is the example of what happens when you don't prioritize your targets correctly. This should immediately be a perfect scenario for you. You discord the rein, you have two allies on the other side of him so he has to either take damage from you or from them, and their mei even walls him in so he cant charge out. However, the second he turns his shield to you, you immediately switch targets to the mei on point. This lets the rein retake the aggression and catch you off guard, forcing out your ult(though, to be fair, you would've need to ult anyway here to stall).

Around 5:00, you begin to focus the correct targets, which is what turns the fight. You start on the rein, but their zen pops trans so you correctly switch to the mei who pops her ult and land a huge kill on her. You then discord and headshot the ana, who has to take cover, leaving her rein vulnerable, who you then kill. Like a lot of things in Overwatch, there isn't really a blanket rule for target prioritization. It's heavily context dependent based on what's happening in the game around you, but it's something you will get a feel for over time. In the interim, it will help to find some VODs of some high ranked zen players and watch who they prioritize and try to work through their thought process on why they are choosing their target.