When I was younger, this happened to me with distressing frequency.
As I get older, I get better...more understanding, more at ease with myself and my partner(s)....and therein lies the secret, if a secret it is.
Be comfortable with yourself...be confident in yourself.....
As you age, generally, your partners get older, too. With age comes experience and that means also with men of size. Up until about age forty, I was almost invariably the largest man any of my partners had encountered. After that, the situation changed and I started encountering more women with experience of size, more women who had had multiple childbirths, and more women who were bored with their marriages or had their hearts set on some adventure, all of which help immeasurably in gaining acceptance. Even if I was still the largest, they were more open minded about trying it - and 99.9% of it all is in attitude....yours and hers.
And, it is also true...if she likes you, if she accepts you...she will do everything she can to let you in, no matter what her degree of experience. It is up to you, through your ease with yorself and your confidence to get her thinking that this event will not be painful or unpleasant.
If she does NOT want you there, it simply isn't going to happen and to force the issue places you in the wrong, not her. It is her body and she lives in there...but if you are reading this far, you probably already know and respect this.
So...chalk the rejections up to experience. If you really want to be involved with this partner, you will find a way, the right way, to get her to accept you. If you are not willing to invest the time and effort, then it is probably better to move on.