You haven't gotten all the air out of the system. It might help if the
group had SOME CLUE as to your car's year and model.

Repeatedly letting it go into the red WILL cause a head-gasket failure,
hopefully this hasn't happened already.

Air lock. Turn the heater full on and open the radiator's bleed valve.
You'll probably need to add more coolant too.

You haven't bled the air out of the system. Do a Google on this on how to
do it for your particular model which you haven't been clear about here.

It sounds to me like you have developed an air bubble in the cooling system.

There is a cross-head kind of screw next to the Fill Hole, this is the bleed
screw. It releases trapped air

1.) Open the screw.
2.) Turn the ignition On, but not Start.
3.) Set heater controls to full hot for both sides.
4.) Add coolant until it comes out of the bleed screw.
5.) Close bleed screw.
6.) Start motor and allow it to come up to temp.
7.) Shut the motor down, wait to cool, top off with more coolant if needed.

Any time a car heater blows cold despite the engine being warm, it's
time to check the coolant level. The heater radiator is usually one of
the highest parts of the system, so one of the first to show a lack of
the warm stuff.

You've got air in the system, turn everything water related on (heater
etc), top up the water, run the engine up for a couple of minutes,
turn it off again, top it up. Then check the level again when cool.

And if you're really paranoid like me, stick a couple of bottle of
water in the boot for good measure.

Dodgy.