My immediate advice would be to move away from barbells for a while and use dumbbells for your pressing movements, so that your pecs can contract using a better range of motion (read: squeeze together more). Another piece of advice I would consider in your case is moving away from an upper/lower split and trying a push/pull/legs arrangement so that you can emphasize your chest more (since it's visually lagging behind your back), rather than doing chest but saving your energy for the back portion. Maybe your schedule doesn't allow for that though...
More than anything else, this sounds like a mind-muscle connection issue, man. Your chest's main job is to move your arm across your body, so instead of pushing the weight up, think of moving your elbows closer together as your drive your arms upwards. That should help move the tension into the sternal portion of your pecs more.
I would also focus more of your energy into "squeezing" motions. So...cable flyes, dumbbell flyes, pec deck, svend press, and hex press (dumbbells touching press). And instead of moving your hands together (during flyes), I would concentrate on moving your elbows closer together rather than concerning yourself with where the weight ends up.
My last bit of advice is to hold your contracted position of any given chest exercise longer. This will, hopefully, force the inner portion of your chest to work harder...giving it more reason to grow.
Hopefully this helps...it's what I did to fix the same problem.