As it occurs to me, it may have nothing to do with your nationality, nor with your alma mater.
Actually, students from Sharif can be very successful in admission polls. It is well-known that they are the finest product of one of the most selective academic systems around the world. Experienced professors are also used to see how well those last perform in qualifying exams, and how productive some of them can be. As a result, you can find former Sharif students in most of the best universities of the world (e.g. MIT, Stanford, McGill, ...) and even more in some specific places such as Virginia, Florida, Australia and Canada. You could even find more of them in States if you were considering the non-negligible part of them who get admits but do not manage to get visa afterward.
Hence, I would rather first rise the question of the intrinsic potential of competitivity of your file, and then put it in perspective with the files of other students from your alma mater that were accepted in the places you target. Coming from a great school with great academic results is good, but if some students in the very same situation than you can claim they were first authors of journal papers, international conference papers, or even book chapters (it does happen), and can provide strong letters of recommendation (as in the terms described by Cup o' Joe), it is crystal clear that they'll be more competitive. Plus, are your Qualitative and Analytical Writing GRE scores OK (they do not need to be great)?