Hare Krishna,
I think you have some incorrect understanding. I have never heard that we get our own universe. We are put on some planet to start our journey of self discovery and purification. Lord Brahma is a very high position, not for some fallen soul. He is instated, appointed to start the universe. The scriptures say that if there is not a qualified Brahma, then He will take the post. It is stated that we have to go through all the species and work our way to human form, then we are responsible for our next life because karma clicks in. There are unlimited fallen souls and unlimited universes, populated by only 1/4 th the amount of souls that have never fallen. Krishna places us here to learn our lesson to not be envious of Him and to purify our misused independence. We can choose, once in human form, to go to the higher planets or Vaikuntha planets, or hopefully, Krishna Loka, so we don't have to go through this again. I hope this helps some. One thing Srila Prabhpuada said was that we should not worry how we got here, but worry how to get out of here.
Sincerely,
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