Is this the onset of mental retardation?

Your self medicating with cannabis you know your not "becoming" stupid, you've always felt stupid, you probably get lost when driving, and unless you have co/existing COD your lack of organisation skills is the reason your rooms a shambles, your not stupid your undiagnosed ADHD.

you seems to underestimate the side effects that weed has on your health ,you should kick this bad habit ,it is possible that weed will cost you your life,weed and many illness go hand in hand ,mental retardation is one of them ,

It has nothing to do with you smoking weed. Weed makes your brain become numb so you do not have to feel anything. There is a reason you began to smoke weed and it may have to do with some deep inner issues of life. Some smoke weed to ease the pain. I was once involved with a man who smoked weed for nearly 43 years and when I left him four years ago it was because of his drug usage. I remember how the weed would make him seem so lazy and have no energy. By the way, this man was 55 when I left him and he was living in an abandoned house and no job. He would work for a man under the table. The weed ruined his life for he once was working and dress so well but the more he smoked the more his appearance started to decline. I bet the minute you decrease the usage of weed the better your life may seem. I feel you have some issues to take care of first so you can get better. Now, ending your life will prove what?

As great as pot is, it's killing your brain cells.

Mental retardation is not developed anywhere outside of the womb. 

FACT:  You are BORN with the intelligence level you will achieve in life and a lot of it has to do with the level of intelligence your parents have and what privilege of education you have had.

And so you were born mentally retarded or you weren't.

Your problem is the weed.  Because you are using an intoxicant daily, your mind is effected by it.

Get off the weed and give yourself a chance to detoxify and after a couple of weeks you may notice a marked difference in your ability to comprehend and retain information.

No. Just that using cannabis has a bad side effect on your short term memory. It's just unfortunate that it has damaged you in that way.  See the info in the link:

https://theconversation.com/does-cannabis-really-a...

What exactly do you do all day? Smoking weed won't cause you to lose the ability to learn things but it certainly won't help you either. The best thing you can do is try to learn new things often and don't just sit around. The more you do something the better you will be at it and the less you will have to think about it. I'm like you though and have to read things over and over if I'm not exactly paying attention to it fully which happens often so don't sweat that, you can still be plenty successful