Certain positions hurt

So I've figured out me and my boyfriend can have sex in most positions... But we can't ever do it reverse cowgirl and then sometimes if I lean back when I'm riding him, that hurts too. If he goes too hard sometimes when I'm positioned a certain way in doggy style, that hurts too... I don't have an issue with lubrication either. I'm always extremely wet. He is big, but damn it's not like I just lost my virginity. I've just never been able to do these positions and now I really want to with him.