Are you using lubricant when you masturbate? If you are pulling the skin too much it can cause streching of the skin on the penis and that can cause some "weakness". It might also be age. As men age, so does their penis. A curve is natural and most men usually do have some type of curve to their penis. Is there a lot of excess skin on your penis when it is erect? If so, you're not using enough lubricant and your skin is streching out. Moisturize your penis to help it regain some skin retention. From now on, use lubricant.
penis doesn't erect straight and not become tight as 2 years back, during masterbating.
I doubt that the veins our your penis can be weakened by masterbation, unless you are squeezing the penis to tightly, and the all it would probably do at the most is rupture a blood vessel. My question is what makes you think the veins are becoming weak? Describe weak?