I've had motion sickness since I was younger (I once threw up from eating a single sweet...), although it's got a lot better. I actually thought I'd grown out of it until I suddenly got it really badly with a car journey recently. Couldn't even sleep because every time I closed my eyes it got ten times worse, so yeah, sleeping is not necessarily a solution. What I find helps the most is looking at the road ahead (except on planes, where it's the opposite and it's looking out of the window that makes me feel sick since the plane doesn't usually feel like it's moving much otherwise unless there's turbulence) and trying not to move my head around/look around too much and just keep my eyes focused on one point on the horizon.
Apparently over the wing has the least movement on a plane, so you could ask the flight attendant if they can see if they can swap someone so you can sit there (or if there are free seats in that area after take off if the flight's not full then see if you can move).
Also, if it's that bad, maybe you can visit your GP before you go and see if they can prescribe you something stronger than what you can get over the counter?