I like imimportant's response. The only way to improve at something is to continue doing it! Try not to get down on yourself, and just keep at it- some things are harder for certain people than others!
Also- I would get the "can't" mentality out of your head. Your brain is pretty powerful thing, and I'm willing to bet that when the time comes for you to so the sit up portion of the workout, you may feel frustrated or discouraged before you even start. Brain power!
If you want another way to work your core, planks are a good option, or "standing crunches" where you would put your hands behind your head and then bring one knee as close as possible to the opposite elbow while standing- but keep at it- you'll improve over time. Even if its a small improvement over a long time, it's improvement!
