Help! SEX IS PAINFUL!

I love my man, we love each other but have issues in the bedroom, we hardly ever argue if we do it’s about sex, I was a virgin until recently had intercourse a few times still painful or uncomfortable, my man has more experience than I, we’re heavier so it’s hard to do certain positions, plus I’m not flexible. We tried missionary, doggie reverse cow girl and cowgirl, every position has been uncomfortable for me, could it be that it’s because we’re both over weight? And as for doggie style we couldn’t line up correctly, plus this man is very quick to cum because he gets so excited, sometimes I have gotten nausea, I’ve tried on my own with a regular dildo still am still just as uncomfortable but if I use it longer it becomes more pleasant but still uncomfortable... what’s happening? Why is this happening? Is because I’m still new at this or what? I love my man and it makes me feel closer to him when we have sex I even crave it from my man if we don’t do it for a couple of weeks I’ve researched and even went to see a doctor and nothing...what’s wrong with me?