I used to have that problem. In fact, for my whole secondary
school life, I spent every morning throwing up (it's not morning
sickness hor!). Went to doctors at polyclinic (trust me, they are
not helpful at all) but doctors just told me there is no cure and
that I just need to swallow my saliva when I want to throw
up.
After my O levels, I used my own method to cure it. I forced myself
to eat. I started off by eating something watery, so that it would
be easy to swallow. After a few months, my appetite became normal.
But I really have to take care and not skip any meals. Otherwise, I
will go back to my old problems again.
