And always aim to stretch “better” not harder. Check out the book “Stretch to Win”
https://mortworld.com/images/stretchtowin1.gif
https://mortworld.com/images/stretchtowin2.gif
You can also lie prone on tennis balls (placed at your psoas) then do a cobra stretch. Other than that there can always be spinal alignment issues that can cause a bilateral difference in hip flexor feeling.
Clear as mud? Oh yea, make sure you are doing your quad stretches too. Also a tight glute can make your leg feel heavy.