gluts: do bridges, but not high enough that it hurts your neck. ONce you get ok with two legs, you can do them one legged

abs: leg lefts on foam roll
Lay on foam roll with arms crossed over chest and legs bent, feet flat on the floor. Activate ab muscles, pushing small of back against the foam roll, and lift one leg at a time for a few seconds. The 'balancing' will work your abs. If you can't keep the abs activated/back against the roll you ware trying too hard, do an easier version or less reps.

If this is too hard, put a hand out on the floor to help you balance (it is the act of balancing that exercises your abs) or start by just raising your heels and keeping a toe on the floor.

As it gets easier, you can stick the leg straight out in front of you or make circles in the air. Since you are stable on the foam roll, it is very gentle on neck and back.

Obliques: lay on floor with legs up in air, bent, toes sort of pointing at a 45 degree angle into the air. If more comfortable place a small (like rolled up washcloth or hand towel) under your butt to raise it. Engage your ab and other core muscles. Lower one leg out to the side towards the floor, keeping the leg bent. Go as low as you can go while keeping good balance. bring it back to center, lower the other leg.

This one feels really easy when I do it but I am always sore the next day. Once again, easy on the head and neck because they are flat on the floor.

good luck, and you are ABSOLUTELY RIGHT...core muscles are the key to posture.

Johanna