Just because you are doing well, it doesn't mean that the other team can't beat up on your teammates. Just recently, I was playing with a group of friends and we were struggling against the enemy team's comp...we had an ezreal bot lane who was getting pwned by the other team's Sivir while we were dominating toplane hard...however, we had to help botlane within the first few minutes. So, we doubled up bot lane with an alistar, forcing them to send another person bot lane...even tho ezreal had been getting beat up badly by sivir (i.e. she was more than 1 item up on him), Sivir + 2nd person wasn't able to dominate Alistar + Ezreal. At the same time, getting a smaller teamfight top helped us out as I could zone out everyone top as Brand and Xin was able to dive disruptively. What happened was we completely marginalized Sivir's advantage and magnified the gold advantage I had on Brand. They were unable to force us to change this dynamic and by controlling the flow of the game, we turned their gold advantage into a loss for them.
As others have pointed out, Heimer is better toplane. While a good defender botlane, he can't turn his advantage into a 1v2 or 1v3 without dying normally...this means that while he might be dominating his lane, they can quickly reinforce botlane temporarily and reset the lane...getting back to top without much ado. This means that all the effort you put in to push/kill your opponent is wasted. Heimer is better top because the INSTANT that you take that turret, you are an immovable wall on that top turret. Diving a Heimer is incredibly dangerous.
Think about how you can turn your advantage into a gain for your team...are you completely steamrolling bot lane but they can send multiple people bot to neutralize you? Consider an offensive switch toplane...You probably have more items than anyone on their team and can use that to your advantage toplane while your opponent in lane was prolly unable to farm effectively, so your teammates might be able to stalemate botlane while you press your advantage top. Be careful with this...if your lanemate was someone like Jax or another strong duelist, you probably need to stay in lane and prevent them from getting back in the game...you are actually holding them down and this is a good thing.
There are many different ways of pressing your advantage, but learning HOW to use your advantage is the difference between Mediocre and Good players...just TRY some things and see how they work for you...eventually you will pick up on some stuff that helps you out.