I won't lie it is a hard game and it gets harder as your score goes up. The asteroids will travel faster and the enemy UFOs get better at evading. Seven is a very good score especially considering the controls are alien to you (pun intended).
My highest score so far is eleven, I think it helps to stay above the enemies because then you have an easier time shooting without getting shot, also you can use the tractor beam which is effective when you are higher especially if you zoom (press d) and are carrying an asteroid or if you wait above an asteroid the enemy is on a collision course with then use the tractor beam while above. You do have to be careful using the tractor beam though because asteroids can be destroyed by other asteroids and they can rise and destroy you like you mention when the tractor beam is used for too long, but to prevent this you can either release spacebar (stop tractor beam) or hold "w" to rise higher before the asteroid is pulled into you.
You are right i probably should have set the controls differently, I am getting a lot of comments about them. Maybe the arrow keys for movement and "z" and "x" for weapons would be better default controls