lol dude if u've allready smoked pot for 15 years then just keep with it.
But if you want to stop smoking pot, then i think your memory might improve, but i dont know about concentration. if i smoked for 15 years then i stopped, i would be concentrating on pot still. idk.

Those things will improve. Also colors seem much brighter and you are more enthusiastic. Good Luck w/ that!

less paranoia and a much happy and positive out look on life.

Yes. Your memory, concentration, energy, motivation, enthusiasm, and endurance will all improve.

It's such a good change to make. I don't know how much or how often you smoke, but if you even just cut back (a lot) you'll see all that you've been missing.

how about getting a decent job,not feeling soo lazy,saving money,being above the influence

Yes, your memory,concentration and energy levels will improve.(Start taking a multi-vitamin tablet along with a B-complex for stress.)
Paranoia will go away.
You will stop craving junk food.
You might even be motivated to do something!

You wont die as soon.

Don't be surprised if you have a literal seizure when you quit. There's no telling what it's laced with. Paranoia, if you're still experiencing it, will be the first thing that goes away. Then, you'll start coughing up tar-like substance from your lungs, which is pretty scary, but it's cool that your body can cleanse itself like that. Memory will improve as well, and yeah, if you're not stoned, you will have more energy.