Taking longer things up my butt

Hi, I am very new to the site, I have been experimenting with toys for a while and is able to take pretty wide things (in my own humble opinion) but I wish to be able to take much longer stuff in my butt to get the feeling of being full but it seem to just stop after 25 cm and while it does not hurt I am scared to keep pushing or pounding in case I damage something in there, I use plenty of lube allways. So I guess my question is if I am doing something wrong, what position is the best to get deep in there. Cheers. All the best.