can't cum

hellloooo. okay so i've been with my man since october. we have sex regularly. he loves eating me out too it's awesome!! the only issue is- i have never came before. like i can make myself cum in the bathtub by using the water from the faucet but i can't any other way!! i don't know what to do and it's horrible. he always is sad after sex because he came and i didn't. what should i do to help? i feel bad if he's eating me out for too long because i won't cum.