This is Dr. David
when you can ejaculate when you masturbate then your nerves and your testosterone levels should be ok
however, during sexual intercourse, you have different stimulation which can cause delayed climax.
there are techniques and sexual exercises you can do with your partner such as mutual masturbation
and seeing a sexual therapist which can help.
if you are masturbating and watching pornography, this can stimulate you quite a bit. and during intercourse, if you are not watching pornography, then, this can cause you difficulty with climax.
Thanks for your answer. I think l will need to see a sex therapist .