It could also be due to your car setup. Certain setups can drastically increase tyre wear, off the top of my head one of those factors is the alignment settings (camber and toe settings). But also keep in mind when you are doing a race in career mode and you've qualified in the top 10, you have to use the same tyres you used to set your grid time. So you're going to start off with an already worn tyre. If you're leaving your tyres to the point where you're lapping 3 seconds slower, I suggest changing your pit strategy and boxing much earlier.