obligatory Sex?

I kinda feel obliged to have sex with my husband. Of course, because we're married. I want to make him happy, but he always finishes so quickly and I haven't finished in months... I'm a student so I am constantly stressed out all the time and I feel a bit like it's a waste of time to have sex if he is the only one getting anything out of it. I know that sounds kind of selfish... my sex drive is practically nonexistent unless I'm on some sort of break from school. my husband want sex way more often than me and he never tries to guilt me into sex or have sex if I'm too stressed out but I feel guilty for saying no so often, we legit only have sex like once a week on average. any advice for people who have ever gone through something like this? we're newlyweds and waited for marriage for sex. the sex itself isn't bad, other than that I rarely finish.