This has only started occurring in the past 4-5 years and I'm 16. On one plane ride, I just didn't eat the right things and there was some heavy turbulence so I ended up throwing up many times. After that, plane rides scared me since I thought that I'd get nauseous on all of them and this lead to me feeling nervous and sick/nauseous even before the plane ride e.g. the terminal, on the way there.
In the past year, that "fear" has passed and I've had a few successful trips where I haven't felt nauseous either on or before the plane. But it also has been happening and I'm not sure why since I'm not one to get motion sick easily on any sort of transport, I don't eat any (large amounts) of greasy, salty etc. food beforehand either. I'm also not worried about my safety when on the plane ride, I just inevitably feel nauseous.
P.S. This is not something I've associated with airports themselves since I can go to airports to drop friends and relatives off.
Please help explain why and what I could possibly do about it since I'm going on a 24+ hour plane ride in 3 days.