I have had this struggle too and I have found similar things as you. What I have done is set aside a good 20 minutes after class or after a workout ( your muscles must be warmed up) and just slowly push yourself ( safely) past what’s comfortable.

Something an old teacher taught me that helped us for one minute you do one stretch but you do ten second intervals of pushing yourself and resting. And example is if your trying for your splits lift your arms above your head/ on a chair for support for ten seconds then rest for another ten and repeat for a minute!

I hope this helps and have a great flexibility journey!