Anal depth: How to increase it and what may be causing my inability to do so.

First of all, hello all, I'm new here :D

Also, this place has helped for a lot of things but not one specific issue. I've searched for a while now but I haven't come across something addresses my exact issue. I want to increase anal depth (through masturbation) however, I can only get about 3 inches in before I hit a sort of wall that is painful to work around. Most people I've read start having trouble around 5 inches, which from my understanding is where the Sigmoid colon begins. But 3 inches seems a little too soon to hit that. What could be the issue and how do I go deeper? I'd like to go at least 5 inches, preferably 6.

I am male, and use a flexible jelly-ish dildo which is pretty thin (not working on width yet) It's shaped like a stack of ice cream (bubble-like). I also make sure I have already emptied the chute. I use a lot of lube and take it slow.

Thanks :)