I’m sorry to hear that you’re having some pain while inserting your cup 🙁
I would suggest trying to hold the cup folded and insert it all the way before releasing your grip on it. This way it doesn’t open near the entrance to your vagina.
If you need to, use some water-based lubricant to help ease the cup in.
If your cup still opens low, you can insert a finger and help guide it into place.
If you’re still having some pain, you might want to try a cup that’s a little softer. You can also try to turn your cup inside out and see if that helps ease the pressure. However, I would only do this if you’re comfortable retrieving your cup without a stem.
Here’s a video that might give you some pointers:
Good Luck and let me know if there’s anything else I might be able to help or answer for you <3