Cross-Legged
You don’t need to be flexible enough to sit completely cross-legged like a pretzel, you just need to be able to get into a seated position with your erection facing up. From here, she’ll sit in your lap, facing you and wrapping her legs around your hips/waist. You’ll achieve maximum penetration and she can be fully in control either riding you or rocking her hips while she holds onto your shoulders for support. Cross-legged is also an important position as it evens the playing field as far as control of the action. You are BOTH in total control.