Sit and face a wall and every day push yourself closer until you're chest is touching the wall. Use the wall to place your hands to keep your back straight as well.
You could try pnf stretching. Basically you engage your muscles in the opposite direction to the stretch for a 3 count, then relax into to stretch.
Work on your flexibility first and improve it gradually. There is no need to tear a ligament during a stretch just to achieve an arbitrary position that hurts your body more than it helps. Start with as far as you can go.
Make sure you warm up before stretching. Lots of hamstring and hip flexor stretches. My favourite is a deep lunge with your back knee/shin on the ground (I think it's also called a runner's lunge?). That stretches the hip flexors, especially if you keep your torso upright and push your hips down. Raising your arms above you deepens the stretch and can also stretch your abs. Then just bring your hips back so you're kneeling on the back knee and your front leg is straight. That will stretch your hamstring on the front leg. Repeat on the other side!
many people look into tightness of adductors as the reason a split can not be achieved. In actual fact, the glutes should be the emphasis as string glutes provide the support as the body gets lower in achieving a split