How do you know how much speed you can carry into a corner? Obviously, this is related to stability and finding the correct line, so when the throttle is rolled on it is rolled on smoothly and continuously throughout the remainder of the turn. But I feel that I'm not carrying enough corner speed for the motorcycle to remain stable. On the flip side of this is that I don't want to go in too fast and crash.

Well this is definitely a 'how long is a piece of string?' type questions; there are so many variables to consider. Surface conditions, tire and suspension conditions and settings, ground clearance all play a part – and that's just the motorcycle and the road. Then there's the rider to consider. What is too fast for one is too slow for another.

Going in too fast can be related to your vision. The less space you think you have, the faster you feel you are going. Space and time are entwined, so if you have more of one you have more of the other.

Ultimately we are looking for a 'in slow – out fast' situation. As you say, going in too fast can – and will – cause you to crash, so ultimately the question is not how fast can I go in – but how fast can I get out. Change your perspective and the answer changes, too.

Cornering speed is only something you learn by experience. You need to practice more to learn it.

Practise, practise, practise + years of riding experience dictates how much speed you can carry into a corner.
Take your bike to a race track as these are safe places to practise.

Keep it rubber side down, safe riding.

I'm not good as well in cornering. But it really depends on the experience and your bike performance.