Most uneven ab muscles in the history of having uneven abs!

Hey guys. I'm 15 and I have quite possibly the worst abs ever. It makes me feel horrible about myself because a couple of years ago I changed my life around and lost 10-15 kg of fat and gained about 10kg of muscle. I'm about 185 cm, and weigh about 77kg. It does sound like I'm fairly fat but a lot of this weight is due to my large velodrome cycling thighs :P . I have a bit of excess skin from the days of me being fat, so to get a visible six pac I have to work extra hard... but now I've hit a brick wall. I basically have a 4 pac taking up my whole stomache, 3 missing (they're there but covered by the excess skin... Need to push hard on my stomache while using a touch of your imagination :)  ) and another one is grotesquly large on the upper most left side (which I would gather would be from going around a velodrome counter clockwise)

When I do sit ups, the 20th sit up hurts the worst because my back is killing me. Am I doing something wrong?
I do 3 sets on and off as well as beating my personal best of what I believe is called the 'bridge' (push up position but instead of resting on palms, resting on blade of forearm). I wake up feeling barely anything, and the burning sensation I do feel is in the 4 abs which take up my stomache, not the small ones which NEED to be worked on!
I've come to the conclusion that now I should do sit ups with a 5 or 6kg dumbell/barbell (not sure which one is correct) every 48 or 72 hours (72 hours seems like the smarter choice). So the smaller abs are absolutly massacred after the work out, but then the 72 hours of rest gives it ample time repair thus [freaking hopefully] evening out.

Please, I just want a visible 6 pac so I can finally complete my goal. It's been 4 years since I set my goal, I've come this far and would be devastated if my abs are forever f**ked.