So far you everyone given you a wonderful advices I suggest you read and understand as they already said contact to the doctor and your first Dom can't be your forever Dom and as you mentioned your Dom is older and experience and you want to keep up with him .. I don't think you need to that it's his responsibility to help you grow slowly and as you mentioned that you don't want to safe word so you take the pain which is not right and it's show's he don't know you well if I know my sub well and I feel I'm pushing her to far to her boundary and my sub taking all pain just to please me I ll talk bc it's hurt me as well safewords are there for a reason if you let him pushing you like this he might hurt you badly which can couse permanent damage for you and even for him.. he might push to deep and your internal body good damage it ll damage you but it also damage him he can go jail for it and many other things ...

And one more thing you need to understand you don't need to ashamed or feel bad about yourself bc we all are unique in our way so is our body as you know there are all size and shape of dick just like that there are all size and shape of vagina maybe there is nothing wrong with you it's your size and depth and there is nothing bad about it ... Don't be ashamed of yourself or your body if you feel that you should share with your Dom and if he make you feel like that then he might not be the right Dom he should be the one who should help you so you don't feel like that