This is where Lee shines because your ganking potentiel and you damage is way to high for enemy (apart from some exeptions). You don't to be sitting around farming the jungle, if you loose your early you will have to get tanky quickly in order to be of some use to your team.

The mid game is when the first team fights starts at drake. At that time your not that tanky you should never go in alone. Dragon's Rage is one of the strongest CC at that time you can just rush to an ennemy Flash, or put a ward if your fast enough, behind the Carry and kick him back to your team and job done, they have no more heavy dps, your team does so it's won. The hardest thing is to stay alive after that, it just all depends on where you are, you can just flash out or if you don't have it use your Q to rush to jungle mobs or put a ward down and go.

Lee sin falls off quite a lot late because his damage won't be as high in the result that Lee Sin base damage on abilities is really high but has really low ratio and must be right next to you to put down his damage this is why you have to be tanky you must trust your team mates to do the damage because you will be here only to throw their ad carry in a death trap by kicking him into your team.

Thx for taking a look at my guide I hope I helped you a lot, please take in account that this is my first guide so if you guys need me to add stuff on it plz do tell me, thx.