While masturbating, you train your mind to ejaculate early,, you will have to re train your mind to delay ejaculation.When you are on the verge of ejaculating, slow down and continue, do this 3 or 4 times before you ejaculate. you may learn male deer exercise fro Google.Kegel is also a good exercise.
You may be suffering from performance anxiety where your mind is kicking in with thoughts of doubt or perhaps you are not totally involved with the situation for whatever reason. It might be better to try 69 position or separately oral or hand job.