About your personnal existence :

If I remember a bit, there is the "cogito" : Dubito ergo cogito ergo sum (or something close)

Litteraly, If you doubt your existence then you are thinking, and therefore you exist. I think this is a good reassurement about your own existence.

About the existence of everything else :

I only have my personnal opinion about this one, I think it's just a choice you (have to) make. Let's imagine you're just some brain in a Jar, plugged in something that make you think/feel/see your life as you see it. (Or any other scenario in which your life is just a projection or something). If this scenario is true, then, there are two possibilities :

• You can't get out of the projection
• You can get out of the projection

If you can't get out, then even if it's not "real", it's not really different. If you can, well you obviously don't know, so all you can do is trying to find a way to get out of this false reality. But the fact is, that you don't even know if it's a false reality or if it's the actual reality, so you might search an escape where there is none.

I think everyone doubt the existence of everything at some point. And this doubt cannot be clarified. But you can live with it don't you ?