Ok first change doctors. Second your dh is never going to reach your uterus no matter how long his penis maybe. The reason being is your cervix(the mouth of the uterus) wont allow a penis through it. All that matters is your dh is ejaculating inside your vagina closer to the cervix the better. The reason I said change doctors is because whoever you are seeing has no clue what they are doing to tell you something like that.
Next step is get the book Taking Charge Of Your Fertility and start charting your cycles. This way since you dont have sex that often you can arrange for the sex to happen when you are your most fertile.
Also make sure you both are eating healthy and you are taking prenatal vitamins or some vitamins that have folic acid in them. It is very importnat for a growing baby. The book I suggested will give you tons of wonderful info on what you should be doing ot increase your chances of pregnancy.