Support question - need help.

Well first things first; taking a break from the game would be a good start, even if it's a short one. Like you said, you're tilted, and while it might not seem or feel like it will effect you, it will.

As for the games, I can't really say. If you're doing really well, and get ahead, the only thing I can recommend is that you abuse that lead. You need to force your opponents into fights, or make them back off and push hard while they're staggered. The longer a game goes on, the more you lose your lead. If you're behind, ward well and play safe; lose some CS over losing your lives. Farm up until you can get a gank from your mid / jg, or until you feel strong enough to start taking on your lane in skirmishes. If you lose your lane, and they don't continue to push bot, a good option is letting your ADC farm while you go roam and try to help your team elsewhere.

It all comes down to good vision control, and taking advantage of mistakes the enemy makes / leads you have. Don't let a game drag out until it's too late.

Hard to say what you are exactly doing wrong unless I played with you consistently. I would say definitely don't play around with your leads. If you are ahead push for objectives over kills. I've been doing this in my ranked games and coordinating my fellow bronze teammates to victory. It's fairly simple, once you are ahead just focus on pushing your lead for towers and the objectives. If you are behind, play passive and punish when you can. Get ample vision our then. Save your cc to protect during these times. I literally played a game as Zyra support tonight (was autofilled). Feel free to look at my match history to see my score for that match. It's all about pressing your advantage. Maybe look up my profile on league and watch the replay of that ranked match.

Let me guess: You mostly win lane. Even in spite of the current paper turrets. Then you put out a massive amount of dmg in midgame, still managing to give most kills to your team (because you are a good Zyra that knows how to avoid killing by mistake) and then suddenly it seems like the enemy are getting the advantage and you desperatly try to control waves and your team starts dying and then the enemy barons and you lose.

From what (others and) I have seen, you seem to be doing a good job early and midgame, but can't close out games. Might need to sharpen up your macro skills, as in when to get objectives, when to rotate, and it can be as easy as "don't get caught out or else you are dead".

As Zyra support, you DO get "outscaled" if the enemy team has more utility, and if you can't end your game fast enough, utility CAN run you over.

Also, a VERY good advice is to make sure your ADC isn't starved on kills/farm/gold early or else the moment he can't catch up (and especially with how much faster games are nowadays), you want to make sure he exists as a teammate. I know it's great being able to carry from botlane when your ADC couldn't, but the moment you need him in teamfights, or even to exist, he almost always have better late game than you do, so you want to make sure he at least has some relevance. A lot of your games have your ADCs being "starved" or "useless", especially if you are his support. Of course because you are a mage and you do need some kills/gold, but you also want to make sure your ADC at least get some of the share.

For reference, I have won games solely because the enemy support starved the shit out of his ADC by taking all the kills, especially when they play as mages in the role of support.

Drop Morello's from your standard build. FQC, Boots, SS, Liandry's, Rylai's or Guise, Rylai's then Liandry's depending on whether you think the slow is going to outweigh the tank HP burn. Consider mobi boots for their roaming power. I usually get Frostfang first, then mobis then finish the build unless we're behind and I know I won't be able to roam.

Level W last, especially with the recent changes to her in the last patch. Personally I prefer to max E as support with 1 maybe 2 points in Q. Midgame your root duration is going to be your key utility.

When you kill your lane early and your ADC inevitably starts to back if you know or have a good idea where the jungler is then ping the tower and push the wave hard. Getting bot tower first is a pretty big advantage.

Other than that the other posts cover it pretty well, press the advantage by taking tower after tower.

The one big thing to keep in mind with Zyra specifically in mid-late game is that since you KNOW that your team will always overstay like every team does use that to your advantage. Even if you see some opportunities to get in some good poke at the tower Zyra's real strength is in the disengage/re-engage. Save at least 1, preferably both seeds and your roots for when the enemy team comes charging down let your team's retreat bait them just long enough for them to get strung out a bit then root/seed/ult the front line. Most times this will be enough for your team to turn, kill them again and maybe even go back and finish off the tower/inhib, worst case scenario you get a good disengage and save them from getting wiped for overstaying.

I'm sure you've seen that last bit play out over and over and those are probably the moments when you see your lead fade away. IMO Zyra is one of the strongest champs at preventing this if you plan ahead a bit and save your resources.