Hello, I’m new here. I operate 2 ATR 72-600s from Cardiff to Heathrow, and I would like to know how to get more passengers to reach above 70% load to break even. The competition is small on this route, and the demand from both is healthy. Any suggestions?
Hello and welcome to Airlinesim
Your question has many answers and then some 
I recommend you read up on hub and spoke systems, first of all, then check out any of the many available tutorial (videos) and the manual.
AS is very dependent on transfer passengers, so having a strong network is important. Also, even if you are the only one operating a specific route, you are hardly ever without competition, as there may be dozens, if not hundreds or thousands of available connections through other airlines’ hubs.
To have a bit more context, which server are you playing and what is the name of your airline?
ATR 72-600 are not the best planes for a this route. I also assume that you have not worked on your seat layout and you are using the default one? That gives you a bad image and makes that you can barely fill your planes.
Also please read or watch available tutorials. That makes it much easier to understand the game.
Good luck!