Hi! I believe I can be of help with this issue.
I can imagine how frustrating this situation must be for you. You are clearly a woman who has insight into yourself and this phobic reaction you have to intercourse and to being touched in the vaginal area must be distressing.
I specified phobic reaction because your tendency to freeze and to stiffen when being touched or when considering intercourse you describe very much as phobias are described. It's important to know that phobic reactions are not necessarily produced by having been molested or sexually abused as a child. Your memories have a large component of fixation on genitals and that area. In classic psychological theory, pre-sexual pleasure was a large part of infant, toddler, and early childhood development. So if your interest in that area was curtailed in "sharp" ways, that could have led to phobic reactions without any molestation involved.
The physical sensation of freezing and stiffening are indicators of such early confusion. The confusion would have been your budding interest in the sensual (pre-sexual) pleasures associated with your vagina and the requirements socially and most likely by family to stop touching or drawing or playing that way. You might not remember those commands and the tensing up that occurred when these pleasures were required to cease. But you describe such tensing up.
My first recommendation is a bit of creative thinking: hypnotherapy. Please note I am talking about only hypnotherapy, which is a recognized form of psychotherapy used by psychologists and psychotherapists. I'm not talking about people who put in ads in newspapers for hypnosis. Maybe those are useful or maybe not, I can't say. But a psychologist trained in hypnotherapy is a real licensed therapist. Even so, here's the important statement about hypnotherapy: hypnotherapy can help with a specific problem and that's why I'm thinking of it. HOWEVER, there are good and honest hypnotherapists and there are other types. Your only way of assessing is two ways: first, make sure he or she is a licensed psychologist. There is no licensure in the US that I know of for hypnotherapy. It's all a "self-licensing" here, which is not good enough. So you want to know his or her license number as a psychologist and call the regional licensing board to make sure there have been no complaints filed. Please do not skip this step.
Now, the next step would be to work on the phobic reaction; but work on phobias is slow work. You need to accept that, okay?
One way is to try first on your own with your boyfriend, who seems to be on your side in this. I'm a little confused about your intentions as you seem interested in having sex but you also seem to value waiting till marriage. I'm going to discuss here what you would do, assuming you are seeking to have sex, okay?
Phobia work is a matter of slowly desensitizing. That means not being quite as stiff and frozen today as yesterday. Little steps. For example, all the foreplay that you like and are comfortable with would be supplemented with small "doses" of having him touch your vaginal area, slowly moving toward it. As you begin to tense up, he would stop and keep his hand where it is. You would breathe deeply. He would say soothing things to you. You might do things that turn you on, all the while he hasn't moved his hand. He then begins to move his hand a little closer to your vagina after you've relaxed a bit. As you tense, he stops, and the pattern continues. At first, he may not reach your vagina. After a while, he may reach it but not actually masturbate you. Then, slowly, it will move to the same with the penis.
You can see how slow this work can be. But that is how phobias are overcome. You can also work with a sex therapist as advised above, but the work will be the same.
Okay, I wish you the very best!
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