Make sure you get some of your practice time on mic. You need a good mic setup with a bit of reverb and some light compression. It's a bit like a dancer using a mirror to check their look: the mic takes your voice away from its immediate connection with the inside of your head and allows you to develop a more objective approach to hearing your voice from the outside.
The ability to listen properly and objectively to your voice is a most important one. Otherwise you would be for example like playing tennis with the other side of the court in total darkness. You would have to rely on how each hit feels in order to try to figure out where the ball went!
You also need to develop the sense of singing within the musical blend, which of course mostly comes out of loudspeakers.
In the mix you use, make sure you have a bit of reverb on the mic, as the sound in your head will mask the simultaneous sound from the PA, but the slight delay in the reverb allows your ear to latch on to the external sound. The compression just smooths out the peak loudness a bit so it sounds more natural.
All of this really helps with singing on stage, singing in harmony, and in recording situations: times when you need to hear yourself clearly and accurately, sometimes picking the sound of of a busy musical mix.
One quick tip: If you are struggling to hear yourself on stage, look at the monitor in front of you. That will direct your ear to the monitor, and you will hear yourself much better.