I am in a bit of trouble here as most people new to shortboards. I keep on stalling.
I catch a wave every once in a while, I noticed that I can't line up with my longboard nemesis now

My pop ups are decent i guess, i can stand on the board, but my problem is here.
I CAN'T STAND ON THE BOARD VERY LONG! I don't know what the problem is or why its happening!After I get on the board about 5 seconds later the next thing i know i'm sinking!!!! I'm somehow stalling the board. Ok so my first thought is my ass needs to get the gas going so i lean forward, soon afterwards my noes of the board goes down and so do I. I've just recently gotten this thing and its my transitional board from a costco 8' softtop. I've also found that I can't ride whitewash anymore.
I'm 5'11 and my weight is about 220lbs. Am I too heavy for my board? MY board is a fish, with dimensions of 6'6" x 21.8" x 2.8"
I'm having alot of trouble getting the board to stay with the wave, i'm not really riding the wave, but i'll catch it then next thing i know i'm in white wash, and it dosn't even stay in the whitewash! on the softtop i could ride the whitewash in!
Anyway I'm having lots of troubles, i'm not really staying on the wave. I'm surfing like waves 1foot plus. I"m surfing at doheney state beach, Laguna Beach, and Salt Creek. I live down in socal. But I need tips, and lots of them.