Flange Size

How do i know whether Im using the correct flange size? Heard that when using the correct size, pumping should be painless, but my nipples hurt alot whenever i pump I am Exclusively Pumping, currently using Haenim 7V+ Breast Pump, comes with a 24mm flange and free 21mm flange insert. First pic is without the insert, just using the 24mm flange. I have short nipples, so the red line is where my areola ends and nipple starts. Can see that alot of my areola is being sucked so I suppose this size is too big right? Second pic is with 21mm insert. But I cannot see my nipple while pumping so how do I know its the right fit? It is still a little painful when i pump with the insert. Third Pic is how my nipple looks like after pumping with 24mm Flange. Forth Pic is how my nipple looks like after pumping with 21mm Flange Insert. Fifth Pic is how my nipples look before pumping. Dont know if the 3rd/4th/5th pics are helpful but just putting it here in case! I have measured my nipples and they should fit 21mm, but the lactation consultant at my hospital after deliver told me I am a 24mm. Hoping to get advice here first, thank you mummies!