If you get the chance, compare the positions (and spread) of the footstraps on your board with the placements on your friend's board.. in relation to the tail of the board, and to the fin. Also, try measuring the distance between the mastfoot and the tail of the board and comparing the two - with your setup, 135cm would be a good number to start with. Obviously, his setup works for you, so making yours as close to the same as possible should help you out.
Perhaps try your sail on his board (or his sail on your board?), and that should give you a clue as to whether it's the board setup, or the sail tuning that's causing you grief.
If you're in the straps, I'm going to assume that you're planing (or you still have the straps in a really far forward, inboard position, and a really funky stance!) It may be that you need to bear off the wind a bit and get a bit more speed on before turning upwind - that will get the fin generating more lift. If you have a bigger fin, you could also try that - I have a 46cm that I use with a 7m and they get on really well.
There seem to be two (at least) distinct schools of thought when it comes to sailing upwind. I am yet to decide which is best as they both seem to work.
First one (and the one I mostly use) is as Red said earlier - rig back, body forward - almost as if you're trying to see around the mast. You hang your weight on the harness (concentrate on taking your weight off the board) and push against your back foot. You have no weight on your front foot, and your back leg is quite straight. If you have enough lift from the fin you can also try lifting the board with your front foot (it'll by mostly airborne, and tilted slightly downwind) so that it flies upwind on the fin.
The second option is to hang way out, and off the back. Driving the board through a straight front leg, and squatting over a very bent back leg so that your hips are almost in line with the tail of the board. I find this quite awkward (I have a lanky build), but it does seem to work, and it is discussed by some of the experts.
Don't stress too much though, going upwind well is one of those acquired arts that takes finesse and patience to master. Sounds like
you can do it though, so it must be a gear setup problem.
Keep at it though, because nobody

wants to sail a longboard!
Let us know how you get on.