but an American tax preparer told me that I would still owe it being a U.S. citizen.
Basically, you're covered by this - ssa.gov/international/agreements_overview.html - you don't have to pay self-employment taxes as there's an agreement between PL and the US on this matter. I'd give the IRS a call directly and ask about it, but it seems to me that you're exempt from those taxes because of the bilateral agreement between the two counters.