How to transiton to Mid-Late Game?

I generally snowball leads for my team or myself and have a really good lead early. However I cant by the life of me Transition into mid game when this happens. Either I suck early and Good mid and win games or Good early and sucky mid and lose games :/ The difference with the 2 is that when I have sucky Early, I try to focus on farm and if I lead, I try to gank more. Is it because of this why I lose games? Also if Im the best player on my team, it means we are gonna lose so theres that too.