Gor a work to be efficient, your soul has also to be strong, stronger than the souls of your friends. When you are more advanced, the advantantages are multiple. You can manipulate a bigger and bigger amount of energy, your mind is stronger which means that your focus is deeper and more effective, which will give more power to the work.
Also, you said your "friends" betrayed you. I would say don't curse all of them in the same time. You would need shitlots of energy to efficiently curse all of them in the same time. Curse them one by one. When you are done with the first one and the results materialised, start again the spell with the next "friend".
If you see that the spells are not efficient, what you can do is to either ask a Demon to help you with this, either to wait until you are way more advanced because you can try the most powerful spell possible, if you aren't strong enough, the results won't be as you desired. But if you are confident that you are strong enough, then wait for the moon in Scorpion or Capricorn and go ahead. Don't forget to create an afirmation which is short, simple and clear, to specify clearly what you want to happen.