Well.
You dominate mid lane by abusing the hell out of Cooldowns. You are an assassin so I automatically assume you have gap closers. Long story short, the moment you see the enemy use any ability on a minion, jump on top of them and wail away. Done correctly, you are virtually guaranteed to win the trade because they (literally) can't fight back 100%, having just used an ability. Rinse and repeat, make sure not to do something stupid, and you dominate lane.
Looking at your account, you play a lot of AD Assassins, which is good. AD Assassins tend to be the best at shoving down towers, remember this. A successful gank means no one is defending the tower anymore, which any point after 8 minutes almost means a guaranteed destroyed tower. Most people say ganking bot lane is the most guaranteed way to pull success for the team in the long run, and it's very true so I don't mean to completely contradict it, but my personal opinion is to help the lane most guaranteed to take down the tower if you help them. A Jax top is probably going to destroy a tower just as if not more quickly than an ADC that fell behind early. At the very least, if you are really scared about being ambushed because everyone's health is low after a gank, at least try to shove 1 wave so that the laning minions start pushing under the enemy tower and are killed off. This helps your laners a butt ton because not only did you probably kill their laning opponents, but you also just screwed up their farm.
Teamfights are somewhat hit and miss because...well, your team. The enemy team too, in fact. This is a gigantic oversimplification but there are 4 basic scenarios:
Teamfight vs Teamfight
Teamfight vs Splitpush
Splitpush vs Teamfight
Splitpush vs Splitpush
All depending on what teams are designed as what.
In Teamfight vs Teamfight scenarios, you want to focus on making picks. Normal case scenario is to go straight for their main carry during team fights, and that works, but another way of dealing with this matchup while even winning team fights before they even begin is to have vision coming out of your eyeballs. What I mean is you want to find every and all opportunities to find that one guy who strays just a little too far away from his team, and kill him. If you can achieve this, for example through warding everything and finding them whenever (this means you better ward too you midlaner), this almost guarantees that your team fight orientated team will now win the next team fight since they are now missing a probably important member of the team.
In Teamfight vs Splitpush scenarios, make sure everybody stays an arm's length away from each other, and the moment you see one of their split pushers especially far away from your team, get everybody to focus an objective. What happens is that if you have a really dedicated Teamfight comp, you guys can get away with a lot of crud in a 5v4 scenario even in dangerous scenarios like tower dives if you play correctly. Now the thing is, as an AD Assassin, 'playing correctly' comes out almost guaranteed if you can at least get your kill fast enough, push the fight even further into a 5v3, and snowball the fight until everyone on their team is dead. What tends to happen is that while the 1 guy is pushing, say, top, you now have (usually) 3 members that survived the team fight now shoving down mid. Who do you think is going to out push? Creating picks also works in this scenario, but split pushers tend to be good duelists, so it's harder to coordinate spontaneous ambushes to go kill somebody you just spotted with a ward.
