All I can give is one years experience and I'm sure there are others here that could give better advice! First off its probably not worth buying a shortboard until you can ride the longboard; the longboard is much easier to catch waves on.
Getting up on white water is easier because the wave is pushing you towards the shore. The mistake I made was thinking "green" waves are the same, they are not. The mechanism is completely different in that you have to use gravity to accelerate down the wave, you can never paddle as fast as an incoming wave either. If the lip is breaking, you can sit up and hope!
You could be pearling for a few reasons, some of the common ones I've attempted are: too far forward, (is the front of the board under water when you paddle?), too far back and catching the wave as it folds over and getting thrown over the top, (how to look like a complete pillock in one easy lesson
), being on the right spot on the board but catching the wrong wave, (ones that dump really quickly so build up and are too steep, where reflected waves meet incoming waves and you get an upsurge of water and others), being on the right spot on the board but because it feels like you're falling, put your hands out in front and grab the rails, (did this a number of times having panicking surfers and swimmers trying to get out of the way as I sped past them and still feel like a pillock
), the problem here is once you have your hands out in front you cannot move so you either pull forward which makes you pearl or you hang on and hope.
What I have been doing (and this seems to work sometimes), sit on the beach and pick a breaking wave, see if the next set are breaking in the same place or a different place. If they are breaking in the same place, thats where you want to be, (make sure the reason the waves are breaking in the same place is not large rocks just under the surface of the water). paddle out and between sets, get to where the foam starts, this is where the next set is going to break, next paddle further out for about 5-10 paddles. make sure you are on the right spot on your board and turn round, (keep an eye on incoming sets as you may get hammered by the odd larger wave breaking further out. When the wave is about 20-30m away paddle towards the beach fast, the wave should pick you up after about 5 paddles and about 2sec before it breaks. You will feel the tail being raised and this is the time to pop up. If you hesitate at this point you wont get it. Ive been told and read that you should do 3 more paddles when you feel the tail lift but Ive found Im slow enough to not need it. Do not look down at the board, look where you want to go. Whatever you do, make sure your arms are at the sides in a "chicken wing" stance, you can then at least adjust where you are on the board and stop pearling by sliding backward.
I hope that helps