Untucking your tailbone: If your tailbone slopes down, as in the first picture, your too-tight hamstrings are preventing natural pelvic floor and glute strength from developing. Follow this step by step guide to squatting, to untuck your tailbone and reinstate the natural activity of the pelvic floor and gluteal muscle groups.
leg day is your best friend :) Squat, squat, Squat, Squat :) and more squats :) Slow and controlled. Maintain proper form. Don't be afraid to ask a stranger to be a spotter. Most people in the weight room are happy to help. If you're too shy bring a friend or just lift lighter weights with more reps. Or use a smith machine. There are endless options ;)