1. If your individual existence is inconceivable prior to your actual existence, then your individual existence is not possible prior to your actual existence.
2. Your individual existence is inconceivable prior to your actual existence.
3. Therefore, your individual existence is not possible prior to your actual existence.

Premise 2: For an individual to be an individual there must be some property that individuates him; a property that he and he alone has. Hence, to conceive of an individual, we must be able to conceive of an essential property that individuates him. It’s that essential property that answers the question, “what were you before you were?” The point being: if we can’t conceive of such an individuating property, then a non-actual individual is inconceivable, or (2), your individual existence is inconceivable prior to your actual existence.

Premise 1: So from the above, my assertion is that a non-actual individual is inconceivable, which entails the claim: only an individual’s actual existence is the essential individuating property of an individual. From which:

1. If that which individuates an individual is its actuality, then that which individuates an individual cannot possibly antedate its actuality.
2. If that which individuates an individual cannot possibly antedate its actuality, (because that which individuates an individual is its actuality) then an individual’s existence is not possible prior to its actuality.
3. That which individuates an individual is its actuality.
4. Therefore, an individual’s existence is not possible prior to its actuality, or (the consequent of 1) your individual existence is not possible prior to your actual existence.

What is the property for actually existing people?

Are you denying we are able to conceive of actually existing people or are you claiming we are conceiving this essential property?

Well apart from the fact that you could replace an individual with a frozen pizza, I am not sure of the point.

If you don't have the imagination to think up an 'individualising' property for something, you can't claim that thing existed before it existed. So where is this double bouble going

This is just ignorance of scientific knowledge (I'm talking about reproduction)... or are you talking about the soul/non-material part of yourself?

yes you can exist before you exist but not under rules of logic or physics or whatever that we use today.

http://plato.stanford.edu/entries/existence/#Ont
Ontological Implications of ‘Exists’ being a First-Level Predicate:
... A prime stumbling block to this conclusion [existence is a real property] is the objection raised by Hume, Kant, and many others that Socrates' existence could be a real property only if it added something to him, which it clearly does not. The obvious assumption here is that, if his existence were a real property, it would have to be like his other real properties such as his wisdom; and since they do add something to him, his existence should either do the same or forfeit all claim to being a real property...

I’m arguing that existence is a real property that does add something to a being; its conceivability as an individual and consequently its possibility.

Much much simpler than that.

To exist means to exist now. There is no existence before or after now. The past or future is merely imagined, but what is imagined is by definition non-existent (though, the act of imagining is of course real).

When you try to prove something about existence then you try to prove something that is already falsified by direct experience, because you're trying to prove ideas, and ideas deal with imagination, which deals with stuff that by definition is not existent. Existence is existent in/as the here and now, it is self-evident and there is no way for it to become any more self-evident than it already is.

Existence is not abstract. Existence is utterly concrete. Its meaning is how it actually manifests this moment, here and now. The meaning/essence of existence right now is "sitting here, drinking coffe, typing on a keyboard". In the entire universe there is nothing more than this simple experience, everything else is imagined, nothing whatsoever is hidden.

^That sounds to me like consciousness.
What is the difference between consciousness and "existing now", as you have described?

No difference. This moment, now, presence, being, essence, substance, reality... it's all exactly the same thing.

But I wouldn't call it consciousness (and none of the terms above), except in an effort to show the falsity of the belief in an objective world beyond direct actual experience, and in an effort to point to what is actually present here and now. Reality is neither subjective nor objective, it is merely real (present) and it is exactly the unified sum of all that is in your direct experience in this moment.

The point of birth.

Here you are talking about the individual existence.
Note the bolded part. Shouldn't it be: an "individual's individual existence" ?
"Individual existence" is something subjective for the subject in your post, while mere "existence" has a broader definition, imo. You can have sort of an existence as a concept when your potential parents are thinking about letting you happen, but you don't have an individual existence at that point.

Did this question exist before you asked it?

In his mind it did.

When I was in High School, there was this smoking hot chick named Debbie. She didn't know that I existed. A few years ago we met at a party and got to know each other. We dated a while, and suddenly I existed.