«وَمَا تَوْفِيقِي إِلَّا بِاللَّهِ»
My success is only through God.
Mā negates and illā introduces an exception. Together they establish a clear restriction of the source of success to God. Tawfīqī is the subject; bi-llāhi belongs to the predicative construction.
The restriction here follows from negation plus exception, not merely from a preposed phrase or general emphasis. A full answer states the construction and identifies what is being restricted to what.