Failing transmission pump tips?

Hi, I have an older car (2007 hummer
h3 5 cylinder base) and since I
bought it it was acting weird between
60 and 80 k/h, always bouncing
between 3rd and 4th gear, I didn’t
know what is going on it was one
year ago... I kept driving for a whole
year and this year had 2 instances of
transmission won’t shift from first at
all and then shifts really hard jerking
the whole car, I had to turn the car on
and off few times before it’s back to
normal.

I am not experienced with
transmissions.

Well I took it to mechanic and he said
sounds like either speed sensor
replace or failing transmission or
failing pump (costs $2000 CAD to
rebuild). So we replaced the speed
sensor to see what happens. Good
for a few days then boom one more
instance of jerky shifts and acting
up...

I have a feeling it does that when it
gets hot too. My thermostat on dash
board won’t show hot, only half or lil
more, but car ground feel really hot to
the touch.. maybe old catalytic
converter.

So what am asking basically is I can’t
afford rebuilding right now as it is my
only car to drive, what can I do in the
mean time to make it last as long as
possible? Drive in D3 all the time? 4
high? I just thought if I don’t use 4th
gear it will help? Don’t drive for
extended periods of time to avoid
heat?

They checked transmission fluid and
even added an additive.

It’s a full time 4x4.