There are two distinct schools of thought on drysuit gaskets...cut, or don't cut.
I fall squarely into the second category. Don't cut your gaskets. Most people can get used to it. If you really can't then stretch it over a small suacepan roughly the size of your neck. Then fill the pan with hot water(NOT boiling) and leave it for a few minutes and it'll stretch out. It's definitely best to start with a small saucepan, then move to bigger IF
you really need to. But be warned, if it stretches too much it'll leak a little when submerged. So try not to stay under water too long
Another option for breaking in yourself
is to buy a replacement gasket and just wear it for as long as you can stand it. You'll get acustomed to it pretty quickly, and it shouldn't take more than a couple of times.
The first time I wore a drytop it drove me crazy. Now I don't even notice it.