cant get my core to strengthen

yea, I'm not an expert but I'd say it's your type of core workout that's probably the biggest problem here, I'm not sure what you mean by upper core muscle? Like upper back maybe? but some good workouts I know that are effective are: lying down sit up bringing left leg vertical touching with right hand and alternate (abs) plank - pushup position with forearms touching the ground (abs) sitting down with legs curled and raised so their not touching the ground and alternate touching the ground with both hands together lying down with legs raised vertical sitting up to touch, as close as you can, to toes with your fingertips (abs) superman - lying face down raise hands and legs above ground so only stomach region is touching ground (lower back) those are pretty good workouts that can be done anywhere since you don't need any real equipment, it's a good idea to link them together too in my opinion, ex. after doing 20 regular situps do 20 crunches, a minute of plank etc... without rest but I'm not too sure what you mean by upper core or if I've helped you out

Well from the sounds of it if your not getting tired doing situps your either built like tyson (2000 sit ups a day) or your not doing enough reps. Everyone has a limit on them, if your not doing them till your stoumach burns then your not doing enough. The best ab workout i have found is sitting in the half cruch, get a friend to throw you down a medicine ball while staying in the half crunch and throw it back, if you can't get someone just throw it up and catch it. You will soon feel your abs burning. Other than that i don't know any exercises other than what Kung has suggested.

These are all good suggestions for increasing your ab workout intensity. I would just add that putting a 25lb plate on your stomach is not going to help. Hold it behind your head or hold it high up on your chest. You are trying to increase the amount of work your abs are doing by lifting the increased weight and if it is just sitting on your abs, it is doubtful exactly how much of that weight your abs are actually lifting rather than just shifting position.

You can also work your abs, not with more reps, but with better quality ones. Do them super slow and really concentrate on the contraction in any exercise. You'll quickly burn them out if you pay attention to the muscle group.

You could also look into certain pilates style excercises. They recieve a bad wrap that they are mostly for women, but they really do develope a strong core. You should look into some of the excercises.

Also, you could try variations on leg lifts (where you lie on your back, legs straight out and raised off the ground, lift your legs to either a 45 or 90 degree angle and then lower to about an inch off the ground. You can also do this with bent knees.

Or, sit on the edge of something (a table, slant board, anything...) lean back with your legs held out in front of you. This should contract your abs. Now, hold a medicine ball straight out from your chest. Try twisting at the waist left and right with the medicine ball held straight out, keeping your legs stationary out in front of you. I hope that is an alright explanation. If you belong to a gym, i'm sure some psycho does these on a daily basis. This excercise is insane. If it doesn't get your abs burning, then you have some crazy core muscles already. Ha! Good luck!