Woo! My period finally came around again, and I laid on my bed and woosh, got the cup in without any problems at all. (Compared to when I tried it the last time at the finishing end of my last period, when there was PAIN. PAIN PAIN PAIN.) I think it's formed a seal (I can't for the life of me twist the cup or get my finger deep enough to feel around the rim), but I'm wearing a panty liner just in case. Also, I can feel the stem poking me a bit, but since it doesn't hurt, and I'm so delighted at actually getting the cup in without any pain at all that I'm going to wait till I have to take the cup out again before I snip the stem off.
Now all I have to do is learn to control the hormones!