New sissy with questions about insertions

Hey all, I've recently begun my journey to twink/sissydom and had a few questions about insertion. I love the idea of being penetrated, but I'm having trouble putting fantasy into action. I purchased a small prostate massager, but I can't get it very far in. I use lube, but it's still tight going in, and I feel like I hit a "wall" that I can't get past. Pushing up against it just makes my intestines sore. Afterward I just feel a dull pain in my lower back and mental satisfaction.

It hurts and I don't know what I'm doing, but it feels so right I just can't stop. Any tips from other, more experienced sissies?