It's likely that you are playing the high notes by simply blowing harder until the octave changes. Using this method, you have very little volume control in the second octave, as you've found, and the highest note you can play will be much lower than you'll need if you want to play more advanced music. It's a very common problem for beginners, and as you get better, it will feel like it improves naturally.
Here's an exercise to work on to fix the problem. Instead of blowing harder, you need to learn to focus to angle of your breathe. Do this by playing long tones. Pick a note, and play it as long as you can. Focus the shape of your mouth to make the sound as pure and un-breathy sounding as possible. Try doing this across a whole scale, if you have the patience. The exact best shape will vary for each note, but you'll have to experiment to find out just what it feels like. The higher the note, the more precisely focused your breath needs to be.
You'll know you've made really good progress when you can sustain high and low notes for equal amounts of time, and control the volume of both.