1999 Honda Civic Overheating

I’m going to try to explain this as efficiently as I can.
My car has had a problem running hot since I got it
a year and a half ago. The problem stops in the
winter and comes back in the summer. This time it’s
much worse than usual. I have to add water to the
radiator and the reservoir before every drive. There
seems to be no water leaking anywhere at all, but
I’m losing coolant like crazy. It makes me think
there’s an internal leak or that somehow the water is
turning into steam within the system. I took the car
to 3 different mechanics and was told 3 different
things. One said there’s a leak somewhere that he
can’t find, another said it’s the water pump, and the
last one said it’s a blown head gasket. I don’t know
who to believe. I don’t think it’s the water pump,
because there is no leak. I don’t think it’s a blown
head gasket because there is no smoke coming
from the exhaust, no milky oil, no oil in the radiator,
no antifreeze on any spark plugs, and no loud
engine. The same mechanic who told me it’s a
blown HG also told me that it wouldn’t make sense
to replace a HG without doing an engine swap
which is another $2k that I don’t have. The car has
196xxx miles on it, but the engine runs like a champ
and these cars tend to last a very long time. Side
note: I have replaced the thermostat, radiator cap,
and cleared out air bubbles from the hoses.
Anybody got any ideas of what exactly the problem
might be? I can only drive about 20 minutes before
it starts running hot forcing me to turn the heat on
full blast.