Hi XXXXXXX

Thanks for posting your query

Masturbation is a natural phenemenon and it does not affect the body anyhow.However it helps to release yourself and make you relaxed and also is safe as it protects from various risks of transmitting sexual diseases if practiced with any unknown partner but it should not be a regular habit as such because it can divert your attention from other important things.Masturbate only when you are excited.
Regarding the premature ejaculation it is due to overexcitement and performance stress.
Some suggestions:
Have lots of foreplay before the intercourse.
Go relaxed to bed.Do not take any stress.
While doing intercourse practice "Start and Stop technique".In this after penetration and few strokes stop for few seconds and then again start.It will increase the time.
Have good diet comprising of fresh fruits and veggies.
Do some form of exercise daily.
Now the weight issue, for that you should consider seeing a Nutritionist who after complete evaluation of your height,weight,choice of cuisine will guide you accordingly and will also keep a track of your weight.Also do some exercises which would help in building muscle but under a guidnce of a Trainer.

Hope I have answered your query.I will be available for follow up query if any.

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