No matter how hard I try, I just can't lift my back up in sit-ups, yet I have many friends way less fit than me who can do it just fine.? How do I do it?
Situps are not the only exercise that can strengthen your core. Other exercises include planks, bicycles, crunches, and inverse crunches. I am not sure your fitness levels but each of these would help you get to a place where you could do a full sit-up. Yoga is also a good way to improve core strength and flexibility as most of the forms work the core.
It may also help to get into some good cardio shape as walking and running inherently work the core. You can try something like MBT shoes (not sure how well these work but they purport to help) that would further stimulate your core during these activities. Either way most physical activity involves your car so as you improve your general fitness your core will get better. If you are looking for form for these exercises either look at YouTube or an app such as NTC.
You shouldn't be doing sit ups anyway, they've been outdated and known to be injury prone since the noughties
Fyi the reason you can't raise your torso is because you're doing it with a straight or correct posture S curve spine. This way you can only come up a few inches not upto knees which is done by letting the spine round.