This is an issue that must be discussed with your gyn in person. You could have a tilt in your uterus or cervix or you may be tensing and not allowing the vagina to dilate as it normally would during arousal. Finding a position is a quick fix but not a cure. There may be some position that would be more comfortable for you in the short term but that does not address the underlying problem. And there is one. See your gyn and let her figure it out.