I skimmed through the other video of yours as well, so I'll be having that one in mind as well with some additional thoughts.
I did see your description note there, but the point stands that you can't play the same way in every game and have the same results. Realize in a duo lane that knowing your limits means knowing your partner's limits as well.
Some of this is simply having a vague understanding of the common patterns of your Marksman. Your Sivir was trying to play the best game for her as well, and with Sivir that usually means pushing lane and poking opportunistically until a tower falls. It's not common for her to hard engage earlier in the game because of her limited mobility and burst compared to other marksman at that stage.
Leona is a more aggressive support, but you always need to be mindful of where your teammates are positioned in relation to you. If your ADC is hanging far back and you're pressed far behind the enemy minion line, for example, they can't join you fast enough to compliment your burst. Sometimes there's not a clear advantage for your ADC to join you either, if the enemy support can simply reciprocate lockdown on your carry, for example. In many situations you would gain a similar amount of pressure and damage on the enemy lane by "soft" engaging with your abilities and then retreating through your own minion wave so that if the enemy wanted to respond to you they are the ones coming into your ADC's territory and not the other way around. If successful, you've softened up the enemy so you can apply more pressure and maybe get the kill on your next rotation. If not successful, you've probably barely been scratched in the process and can more quickly resume your assault.
Being aggressive is not a bad thing and how aggressive you want to go in some part depends on how it suits your playstyle. You are correct in that it often lets you control the flow of the lane. However, there's a difference between being aggressive and being risky. Here are some examples that cross the line:
-Giving pursuit far into lane past the enemy wave.
There are a few times in your games where I see your allied carry go from something like 3/4 health to 1/4 health without ever taking a hit from an enemy champion because they have followed you past the wave.
-Diving turrets without overwhelming odds (Team health/mana pool is dodgy, allies not in great position to follow up.)
Every time you've done this in the game I've watched you've given up an equal amount of deaths or more to any enemy you've taken. If the enemy is that far back past their turret, just take the turret instead.
-Invading the enemy jungle without overwhelming control.
This is a common pattern across both of your games. You pick up an early kill in an exchange, and instead of using that time to regroup, secure control of the enemy territory and push an objective, you pursue low health targets well into enemy territory and maybe kill them after 10-15 seconds of playing tag. Most of the time when this happens the enemy jungler and other laners come around and smack a kill or two off of you in exchange. You might or might not come out ahead in the kill exchange, but nothing else is happening on the map all the while.
There's a common theme in everything I have said to you thus far. Define being aggressive to yourself as applying pressure, definitely pick fights more wisely.
You don't take the time and patience to stack the odds in your favor quite as often as you go straight for the throat and you waste time with a lot of these tactics trying to secure kills that end up letting the enemy team set up to counter you.
An even trade with the enemy is a waste of both of your time.
I'd like to mention a couple of your good points, with some advice to temper them, of course.
Based on your other game, you do appear to have a basic understanding of the importance of vision dominance and objective control. However, you seem to often treat these as secondary elements to pay attention to once there are no more blokes within sight to kill.
Really, you can brute force your way through a lot of these issues at low level play, and for example the game you had with the Zed afk you probably would have won through brute force scaling if he was there. You could have closed out about 10 minutes earlier if your team had spent more time killing towers and inhibitors, though.