This is very very common when starting out, you sound like you're describing a lot of my early surfs!
Where are your hands when you pop up? Are they nearly midway along the board? If they're in the front half they will be pushing the nose underwater, get them back by your ribs!
What type of waves are you surfing? Are you still on whitewater waves? That's where we all start and I was on them for about 8 months, lack of a proper green wave means you're catching white water which has lost most of its power, standing up on that is like balancing a spinning top that's spinning too slowly.. It'll soon fall over. In the whitewater your speed and impetus drop off rapidly as does your ability to balance because the wave is losing power.
Going too straight can shorten your run and mean that you quickly get to the point where its next to useless trying to stand up. My advice is forget standing for a wave or two.. Try catching the wave then angling your board to the direction you want to go, slightly across the wave, say 10 degrees, not much, you'll feel the board plane along the face of the wave and it will be smoother and steadier, it will also increase the length of your ride, do this till you can do it every time, then try and stand but get to your feet early!
Speed! The less time popping up, the less time for things to go wrong, practice on land, practice on your bed (I like this because it simulates the non-hard reaction of water as you apply force) learn good technique! I saw a middle-aged woman at my local popping up in a split second, you can't tell me she was stronger than me, it was technical ability.
Arch your back first! Then swing your legs between your hands as you lean a little towards the nose, using core strength to bring your legs up to your abdomen, if done at the right point on the wave the tilt of the board will help.. Do NOT have your hands out in front of you.
Make sure you've actually caught the wave before standing, most people starting (me here) start to stand as the wave is dropping them off the back or petering out and has lost power. Putting too much weight at the back of your board will sink it and acts like a stalling device. An ungainly and beginners pop up usually involves a foot resting on the back of the board for balance and stalls it, resulting in a quick dismount there after.
As was mentioned a big board helps, I'm 6'3 and have learnt on 8 foot boards, I don't see that you would need any more but try a few out?
Take a few lessons, as much info as is on here and as wise as all the gurus are, no one here can see you so get a good
surf instructor and listen! You've already paid for a camper van, 30 quid or whatever ain't gonna kill ya!

Practice practice practice, we've all been where you are mate, it gets way way wayyy better

you're gonna love it! Part of what I love most about surfing is the feeling of progress. You'll be fine.
Most importantly ENJOY!!!
