I am going to look at your most recent game. Take everything with a grain of salt, expect the runes that i 100% right about
http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2206609141/231488968?tab=builds
The correct build can improve your chances of victory by being stat efficient.
I think you need to clean up your runes. You are still using tier 2 runes and not all your tier 3 are efficient. Red pen are way more efficient than AP.
masteries i think you should jump out of the resolve tree to ferocity. You may want to look into death fire instead of thunder lords. thunderloards can only be proc once x amount of time. This is inefficient for dot mages.
gameplay
Your early game is pretty good. You are not dying and taking the tower. You need to know how to ward and place mushrooms for optimal split pushing. Teemo is not a good team fighter so you need to learn how to be that person that draws 2 players to deal with you and you kill both of them off.
you are not buying pinks. You need to buy pinks. It will help you split push when you are ahead. It will help you to know when to base or run away when getting closed in on. Your ward count is low considering you never changed trinkets you need to ward smartly and not rely on just your mushrooms to keep you safe.
I could make some blind assumptions from this point on but not would not be really helpful. work on being optimal it does become secound nature over time.