1. That there was a creator of life that predetermined what our lives should mean.
2. That no other meaning except for the one described in point 1 is valid.
It doesn't work that way, meaning is not singular and is not preset. A "meaning of life" sounds like a rational idea but you can't condense life down to a single meaningful goal without losing its essence. Life is what you make it.
Why would a god that needed us to fulfil a singular purpose give us the power to choose to not fulfil that purpose? Why would a god choose a purpose and then not clearly communicate that purpose? Why would a god give us the capacity for reflection and awareness, allowing us to derive our own meanings, if none of those meanings were valid?
What would a single meaning of life even look like? Let's take a guess and say that love is the meaning of life. That would mean the only point of life was love. All the other experiences? Meaningless. Says who? Who says they're meaningless?
"A meaning" is like "a sock". There is only one true sock, all other socks are false, only those that have this sock can truly know what a sock is, etc... You're trying to condense a potentially infinite group to a single example.
As for the whole "beneficial to us" thing, in what sense? The whole "life after death" thing? Something else?
Also, if you accept that any possibility is potentially the reality in which we inhabit, then it's possible that you are god. Now you could be god, what do you want to do with your life?
The possibility of solipsism cannot be excluded, but it also seems to be very difficult to test. I don't want to spend too much time thinking about it, because there are two approaches of broadening our knowledge which appear to have lower hanging fruit: The theory of mind and the theory of everything.
A universe without a creator might seem scary to those who have been brought up to believe in a creator, but how would it affect your day to day life? The pleasure and the pain found within existence is just as real as it was before, you can still take time to reflect on the nature of existence as it's a fascinating but unanswerable question, the only real difference is taking advice from different sources about the path you take through life.
Furthermore, if there is a God, and this God designed us, then our natural state is what God intended us to experience. By separating ourselves from the life that surrounds us we can disconnect from these experiences. How can a God judge you for living according to the nature it designed?