The game is balanced so that you should survive even with competition on every line, not with big profits, but with small profits. If you make no mistake. Of course, it's harder than when unopposed. But I've made money on routes with 2 opponents suppliying the whole demand. Each of them. I was not making a lot, but still. There are a few things you can try next time:

(1)frequency. Fly more smaller planes. That 100NM route had 1000 demand, and opposition were flying 10 daily A320 flights each. I was flying 15 E175 flights. With exactly one hour between each(below 1 hour of interval, you begin to have problems) I therefoer had a better LF for my planes.

(2)price. Lowering your prices sometimes work, sometimes not. Take notes, and see what works. Try to raise, try to lower, and see how the plane profitability changes.

(3)Leave. try to find another base where opposition is not

(4)plane use. still on the same 100NM route, my planes are flying 5 times a day each. For the smallest ones, 6 flights a day is possible. In current GW3, I've got a SW3(a 19-seater!) that makes 250k$ a week, thanks to its 6 daily flights. Substract 60k$ for the personal fees, and you'll notice this plane is very profitable. Fly the same 6 on a 120-pax demand route(below 280NM) where your opponent landed a A320, and see him cry.

(5)fleet groups. Try to have at most 2 of them. focus on the lines there are good to.

BTW, can you tell me an example of line where you've been attacked? What distance? What demand? What plane did you put? What plane the opposition did put?