You should not stretch until it 'hurts like hell'. Just stretch until you can feel it. If you stretch until it hurts, it'll hurt next time you try and stretch, and the time after, and the time after and then you'll have an injury on your hands. Regular comfortable stretching is the way forward.
I couldn't disagree more. You'll never see a fundamental change in your flexibility without deep stretching, and deep stretching hurts.
Again, though, don't bounce into it and don't just force yourself to your max without warming up and properly working your way up to it with lighter stretching first. This sounds like the actual cause of the injury problem you're describing.
Weight training hurts. Intense cardio hurts. Stretching hurts. Pain is a part of progress, but just because you feel pain doesn't mean you're progressing. Everything has to be done intelligently or yes, injury is inevitable.