aarman is dead right on this.
Stick with the exercises you have been doing, since you have noticed progress.
Excessive masturbation in a physical sense will only cure your problem, not make it worse, however if you are getting more obsessed to the fantasies and arousal feelings that there is your problem.
Along with doing physical exercises you should be working on controlling your mind and arousal state. Try to masturbate without visualizing anything, ditch the porn, and focus only on yourself.
THEN Slowly, work your way up to introducing exiting things, and work on Stopping your arousal. You can do this in many ways, for example, stare at a hot naked chick for a few seconds, and then do something or look at something that turns you off. Basically you need to retrain your mind to not get so excited, and calm down. Try some relaxation techniques.
It's a mind control thing. You can do it. You just have to control your emotions and thoughts.
I just posted this a few minutes ago on another post, and it might be of help to you
A few tips for raising your endurance levels in sex, and conquering premature ejaculation:
It is good to lower your performance anxiety levels, such as calming yourself down before hand. Anxiety is NOT good for what you are trying to achieve. This also includes arousement. If you are TOO aroused, it will lead to ejaculating too fast. This means you have to start practicing exercises, like meditation, or whatever helps you to calm down. You can learn to withhold from ejaculating with practice (lots of practice) but it's possible.
Stop THINKING. If you are worried about the future of pleasing your partner all throughout it, it's not going to make things better. Learn to masturbate without thinking or looking at anything for starters.
Also check this out: Not all women think it's a bad thing. I for one, think it's hot. You can always go at it afterwards. Maybe you should find a girl who is big into ejaculation, there are many out there. You may not believe that, because in society it's looked at as a bad thing. But, it's not really a bad thing. I have a lot of sex, i'm quite experienced, and from personal experience, i think its really hot, as long as the guy wants to go again afterwards.
You have to masturbate more. It's sometimes caused by not enough sex or stimulation. This just means, practice, practice, practice. It's not just a myth. Masturbating is healthy, and it does increase your endurance levels. Not having sex enough, or not getting enough action, makes you more prone to getting extra exited and aroused, so make sure you exite yourself on a regular basis. Maybe what you need is to have sex, and not just masturbation.
Try watching porn, or something that excites you, and try NOT to get exited.
You might also look into upping your confidence. Sometimes low self esteem can play a big part in it.
It would be wise for men to work on muscle tension levels, because muscle contractions, holding your breath, or muscular tension can lead to ejaculating too fast. You can work on this through mindfulness, yoga, or stretching, or before hand doing a hard workout. Pay attention to what your body does during masturbation and orgasm. Try to fight it.
Try the start stop method, which is said to work. Masturbate until you feel it coming on, and then stop. It can feel like torture, but it gains you more control.
Mostly, doing it more, usually ups your endurance.
I am sure there are more tips i havent mentioned...
Good luck! "