Something which cannot be created has existed in eternal past. Let’s go back to eternal past. We still have eternal past in back and eternal future ahead (this is the property of eternity). We observe that the thing exists in eternal future. Therefore the title is proved.
We need to discuss the status of potential, actual and necessary. Potential, it seems, does not exist in the way a chair or a stone does.
I believe this bring us to the “sempiternal” of the ancients, in Plato and Aristotle for instance. The Catholics remove God from existence, through the claim that the actus purus or (energia in Aristotle) differs from flawed existence through its freak perfection and creative power (remembering, that until quite recently, creativity was said of God only, since humans invented or discovered what was there in potentia).
There is the ‘past’ ‘present’ and ‘future’ and these are all human concepts and realization.
The term ‘eternal’ is not a human concept.
‘Eternal’ is merely a mental thought that has no possibility of being real at all.
Humans can discuss the concept of past based on memory, the present based on current experiences and the future based on speculations and predictions then compare to the future present.
The idea of ‘eternal’ is not a possibility to be real at all.
Thus when your premises are imputed with an impossible element, your conclusion will follow with an impossibility to be real.
If something has existed for an eternity, than it’s already been subjected to everything it could possibly be subjected to, and survived, so there’s nothing anything could subject it to in the future it hasn’t already been subjected to the in the past, and survived, so it’s indestructible.
But not perhaps as time. IOW we experience change and call it time. But change may be an illusion. Perhaps it is a block universe with time as a fourth dimension.
There are two main contentions within philosophy regarding time, i.e.
time is independent of humans and
time is co-dependent with humans.
Common and conventional sense wise, it is very obvious time is independent of humans.
But with philosophical reflection, time is not independent of humans rather time is co-dependent with humans.
Time emerged from the consciousness of human beings.
Without human consciousness there is no realization of time at all.
Time is not something that has pre-existed and waiting to be realized by humans.
Time is co-dependent with human existence.
If is not possible for time to stand alone by itself without being complemented with human existence.
Your first premise is too messy and contained too many unjustifiable elements.
“something which cannot be created”
as long as it is a thing, it is co-created by humans.
there are no things which is never co-created by humans.
thus if something is not co-created, it would not have existed.
An uncreated thing would not have existed in the past.
I have also an eternal past is not tenable.
It would be easier for you to start with the argument or thesis;
Thesis: Thing-X exists but is not created.
Prove this thesis first before you proceed with other thesis.
There are two types of time, psychological time and physical time. The existence of physical time is part of general relativity which has been shown to be true recently by gravitational wave experiment. Physical time exists independent of human, it curves, etc.