Sex hurts - how to stop it? Please help!

I lost my Virginity six months ago, however, I still experience discomfort. My boyfriend's penis is 8 inches. How can I minimize the discomfort? We do use a lot of lubricant, and we do participate in a lot of foreplay. However, I still feel pain and we have sex regularly. I've worn super plus tampons since I was 13 (19 now) and have never had an issue with the insertion and removal of those. Also,  I've used a diva cup and that's also easy use. I've never used a dildo/vibrator but I can insert a few fingers comfortably. Is it because he is too large? We often have sex in the military and doggy style positions but neither makes penetration feel great. I only seem to experience clitoral orgasms. I want to have good sex, but it's like my muscles won't relax even though I am VERY comfortable with him. I find my body will subconsciously try to push him out of me. I do kegels. Is that an issue? Are my vaginal muscles too strong? What do you suggest I do? Thank you for any advice!