2000 cavalier coolant light on after thermostat replacement (a 6 day project)

Ok. Bought a 2000 Cavalier with 60k on it;
never maintained (oil changes only). Low
coolant reservoir dash light came on so I
just thought "Meh I'll quickly chanhe
thermostat".  Oh, how stupid I was... if
anyone reading this is even remotely
familiar with cavalier thermostat housing
they now exactly what I got myself into
here...
Day 1... unable to remove lower/rear
thermostat housing bolt.... after 2 hours of
failed attempts i removed the entire
housing.
Day 2... After using a blow torch and every
other tool known to man i gave up on trying
to get that bolt out... after a 2 hour break I
decided I was gonna remove that MF if it
killed me.. ended up bending the thermostat
housing cover all to hell, but this enabled
me to FINALLY remove that bolt.

Day 3... With thermostat housing cover
removed I went to get new thermostat. I
thought "Woohoo, I'll be done in no time"...
while standing in line at auto parts store I
noticed a GIANT crack in the housing (not
the cover)... so I decided I'd just buy a new
one. NOT.  This part has been
DISCONTINUED by manufacturer. (at this
point I asked auto parts clerk to just shoot
me... he declined). Spent the rest of this day
searching for thermostat housing online...
eBay and any other possible used auto
parts site... EBay etc etc..

Day 4 Entire day spent walking thru salvage
yard. Part found. 2 hours removing this one
as well (didn't bring my wobbly extension)

Day 5.... spent this entire day getting the
&%$#! bolts of the salvage yard thermostat
housing (very carefully... didn't want to
crack this one too). Finally got them off....

Day 6 Spent 4 hours trying to reinstall
salvage yard housing... without wobbly
extension. At 7pm I gave in and biked to
Home Depot (9 miles each way) to get
wobbly.... got home and got housing back
on in about 6 mins (yeah yeah the wobbly...
I know.. I know)!

Day 7... everything back together, coolant
carefully added, burped.... turnend the car
on. The good news is that there aren't any
leaks (not bad for a gal)! However. Now the
heat doesn't work and the dash light is still
on.
What exactly is the problem? Clearly it's not
the thermostat...
I did buy a new water pump but didn't
install...

I'd really appreciate it if you guys could get
me through this one.... I'm a nurse
anesthetist NOT a mechanic ( but I do like
fixing my own cars my tools).... sadly, I
don't have any actual experience so I just
make it  up as I go along).

Out of my tree and teetering on the em edge

Thanks!