For me the key to the answer is that there is only exactly one person to whom God can show Himself so that I have no reasonable doubt left: that's me.
There are people who bear witness that He did so to themselves. This is good to know, but couldn't possibly solve my doubt I take it as quite too serious and consequential a difficulty to judge on hearsay (of strangers) only. So if you happen to be one to whom God did not (yet?) reveal himself there possibly isn't anything else you can do but doubt His existence.
I think the only one who'd be able to answer the question is God himself. (By which he'd reveal himself to the OP "beyond reasonable doubt" I think...)
I'm not aware of a promise that He'd do so to a given person at (before) a given point of time (before the end of the world).
This makes me wary about some points that came up in the discussion above:
(Disclaimer: really comments would be more appropriate for the following, but I'm new here and not yet able to comment).
I'd like to pick up Bork's quote and emphasize two further half-sentences:
Rom 1:18-20 NIV The wrath of God is being revealed from heaven against all the godlessness and wickedness of men who suppress the truth by their wickedness, (19) since what may be known about God is plain to them, because God has made it plain to them. (20) For since the creation of the world God's invisible qualities—his eternal power and divine nature—have been clearly seen, being understood from what has been made, so that men are without excuse.
Seems to me, only someone who knows the truth can suppress it. In other words, this is about consciously rejecting God. Also the 2nd part makes statements about people to whom God has made plain what can be known about him by humans. As I understand that it simply doesn't apply to people who did not yet find God (as I see it, it would be more preciset to say: whom God did not make find himself / let himself be found). I do not suspect the OP of posting a misleading question, so I assume that he didn't experience anything like God revealing Himself to the OP. So IMHO these verses don't help answering the question.
By the way, I take a similar view about the blessings: If Jesus states that if you believe in God without having seen him, you are blessed, that
- doesn't say you cannot be blessed if you didn't see him
- neither does it say that you forfeit the blessing if you ask for a personal proof
- neither that you won't eventually get your personal convincing proof
- but neither is there a promise of such a proof.
- actually, I think the blessing by God (grace/mercy) produces the faith (necessary condition!)
So taking up the coffee metaphor from above, people tell of cups of coffee being handed out, but it looks like the OP didn't get one so far. Which may be encouraging the OP to ask for one for himself, but unless he gets it that is not all that helpful.
I personally don't even know whether I ever did get one (if I didn't I do reasonably well without the coffeine, if I did, I didn't realize it was coffee), but iff you ask me to I could try to pass on the word of your asking for your personal cup of coffee. I don't think that is necessary anyways because I believe they know if you'd like to have one - whether or when they serve you is a completely different question, though. You'd need to tell me, though: trying to get a coffee for someone who doesn't like it in my opinion is not only useless but positively rude - so I'd never do it. ;-)
 Personally, I happen not to doubt His existence.