You should talk to another doctor. Your doctor should be concerned and offer you options when you come to him/her with any kind of pain in the vaginal, uterine area, especially if they can't find a reason for it. He should have at the very least offered you suggestions on things to do that would help make sex more comfortable for you. It is a vital part of your relationship, marriage, and part of who you are. You should be able to enjoy it and don't settle until you find somebody who is willing to help you find a solution. If you don't want to talk to another doctor, you could try massaging it as Rebecca suggested... or try the physical therapist.
I had a second degree tear, but sex was painful until my son was about 10 months old. I talked to my OB/GYN. She didn't see any reason I should be in pain during sex and thought that perhaps I was causing it to hurt - i.e I was expecting it to hurt, so I would tense up and that would make it hurt. She suggested the type of massage Rebecca suggested to help. She also suggested I try lube since I was nursing. Having my husband massage the scar area (as well as the rest of me) helped me relax and since it was part of the foreplay, I also became aroused. It only took a few tries after we started that and I stopped tensing, and it stopped hurting. So, that's a possibility. By the way, I use "Just Like Me" lube by slumber parties - best lube ever, never gets sticky. The ones they sell in the store have never worked well for me.
I also have to say that my scar behaves just like any other scar and once in a while my scar still hurts. Sometimes its during sex, sometimes I can just be sitting, but it does. My sister says the same thing happens to her. But we both have large scars on our knees that have been there since childhood and they periodically hurt too, so maybe we just have some weird psychological thing going on, I don't know, just thought I'd put it out there.
In response to Kris S. who said "...there is no substitue for natural lubrication. Natural lubrication indicates that you are enjoying yourself, you are relaxed, and you are ready. If you are not truly "excited" it doesn't matter how much tube-lube you use." I agree with you that it doesn't matter how much lube you use if you are not excited, and that there is no substitute for natural lubrication, but lack of natural lubrication does not necessarily mean you are not ready for sex. Some of us can get aroused and yet still need lube. I myself need lube because my excretions are too thick when I am nursing and for about a week after my period before I am fertile again. There is nothing wrong with using lube and nothing wrong with me medically, that is just how my body is.