Never go deeper that she is comfortable. You don't know how she is inside, what condition or structure in her rectum and large intestine is causing pain. Forced anal penetration can rupture the intestinal wall. Do you want blood running out of her butt while you try to get her to the emergency room? I know it feels good to have your balls slapping against her butt, but I would be quite satisfied to feel the head of my cock being loved by her tight little asshole in repeated insertions and withdrawals.
As said above..to add you can stroke her with your finger (or fingers) to warm her anus up. It is easier to go deeper with the finger. Also a great addition if you lick your finger everytime you pull it out of her anus. There might be some feces stuck but if you really love her you would love to do it.
Just go slow and let her relax. It is easier with time. We go now balls deep and sometimes really hard and fast. But it takes time and practice.
When you hear people speaking Chinese turn around and go back where you started. Then you can do it again.
No need to be rude. If you do not like something it's ok. I do not judge you that you are puritan, inexperienced, or anything else. In fact I do not really care what you like. We can share opinions on forum why we like something or not but it is just for sake of exchanging experiences not to convince someone to something. If someone else likes it is ok as well. It's each of us own business. In sex there is no "normal". Each of us is different. For example we like sometimes slow, delicate vanilla sex - and sometimes we do various "strange" and "hardcore" things. The clue is that we both like it. Take care.
How deep can you go? I say go until you feel the back of her teeth lol No really though respect when she says stop because there is a reason for it.