I'm not super high elo or anything, but I've played my fair share of support for more than a fair enough amount of time.
TLDR; Look towards playing more proactively and for your team instead of yourself.
At first I was just going to say that you should just play less and things will work themselves out seeing as you've put in about 700 games the previous season. Then I decided that I could just watch a few of your matches to compare myself to you to see how much I differ from someone with a higher rank. Granted I only watched a few of your matches so this may be a bit harsh and off because I've not seen enough of your play style to fully evaluate it.
First and foremost the excuse of, "Oh, but it's high elo you can't compare..." Yeah, no. That shit don't work with me lol. When I watch these games all I see is a normal Silver game. People running around letting the game play them instead of playing it.
Your play style is completely backwards. Especially on a champion such as 
. You play passively during the laning phase, and then switch to full aggro during mid game and when you roam. Your laning phase should be way, way more aggressive to the point that the enemy laner will want to jump on you or run away from you every time you step forward slightly. You miss out on so many gold procs just because of that passiveness. You also have a strange obsession with chasing people down. This is what leads to a lot of your deaths. This also affects your positioning a lot of the time as well. You either get caught out because of it, or your ally, especially your ADC, pays for it. You should work towards making it so that your positioning is more favorable towards your carries. You're also too greedy towards giving yourself damage. A good example is when you play Nami. You use your E on yourself way too much. A lot of the fights that you lose and kills that you drop are simply because you prioritize yourself over your ally. You simply could have turned and won a majority of lost fights and kills if you had used your E on your ally instead of yourself. You are after all playing Summon Aery which gives a bonus shield to them. This in turn extends your ally's overall durability while granting them damage.
This goes on into my next point of your thought process being a little lacking. You're too focused on, "I've got to play for me, myself, and I only." You should be more focused on playing for your team and how you can help your team push their lead and recover if behind. I'm not saying become Stalin and go full dictator, but you are the support. As the support you can heavily dictate the flow of the game just as much as the jungler does. Instead of doing so you just let the game play itself out and then react accordingly. If you want to further excel as a support you have to push games in your favor. You have to play proactively.
This next part will be 2 parts.
Your build pathing is okay, but a lot of the time you spend your money poorly. You're using more control wards than necessary. Control wards are nice, but you have to remember about placement. Can it be protected? Is it worth protecting? Will it last long enough here? Buying more control wards than necessary and just spamming them about slows down your overall build. Again, because you're letting the game play you and not the other way around.
Boot rushing and finishing your support item. These pretty much kill what momentum that you've built up. Especially since you don't play proactively. Mobi's IMO are terrible on Nami. You've so much MS already that most other supports don't have the mobility to do as much as you. On top of that Nami is a support that you will poke/fight a lot on so they don't do much. Lucidity is a much better buy for the 10% CDR all around as well for summ's. You only need the support item on T2 so that you can get the warding since the gold passive is the same at T2 and T3. Sure you get more stats, but for what? You're going to waste money on an item that's already as good as it gets instead of putting it into an item that does more with the same if not better stats? It's a no brainer.
Runes and Summ's. First off is Ignite. Because you're not cheesing anyone let alone playing proactively there's really no point. Exhaust is a better option because you'll always have more potential targets. No further explanation needed. Magical Footwear is okay, but it's not good. You're better off running Future's Market which grants you better gold viability. It only tacks on the extra 50G per purchase. Meaning if you don't buy in multiples you don't lose out on gold. Celerity is fine, but again you're not playing proactively so it does nothing for you. You're better off running Transcendence to get the extra 10% on top of the bonus AP when you pass the 40% cap. This works for both you and your team.
