Hi Everyone,
I started getting back problems when I had a knee injury, I think due to how I limp. Anyway you may want to try this book, as that helped me quell the pain a lot, and then you should find the exercises easier. It should seriously increase your flexibility in tight areas. It will also help you guage which areas actually are tight!

The Trigger Point Therapy Workbook: Your Self-Treatment for Pain Relief Trigger Point Therapy Workbook: Your Self-Treatment Guide for Pain Relief: Amazon.co.uk: Clair Davies, Amber Davies: Books.

Also when you do things like situps etc. possibly try to engage your abdominals from the pelvis by using the 'Zip' technique Bruce was talking about. There's a lot of information about that sort of movement on the Pilates 'about' page. I am no expert but it sounds to me like the bridge would be a good exercise for you to do. That should help you engage your glutes. However check them for trigger points first in the book I recommended. If you don't want to purchase the book then there is also plenty of information about them on the internet. Though I don't feel much beats that book that I linked to, but I'm sure there are some hidden gems on the internet. If you have bad hip flexors you may want to start here on the trigger point side:

ROUND EARTH PUBLISHING: Knee Pain: Quadriceps: Rectus Femoris

Also check your Psoas for tightness.

Best of Luck.