Voluntas suprema lex mea
This could be interpreted different ways given some ambiguity of what "mea/my" referes to.
"My will is the supreme law" is legitimate, but given the word order, it could easily mean "Supreme will is my law." In that case I assume that it would be referring to God's will.
Two opposite meanings (unless god is the speaker!), but I think either is technically correct.
I quickly googled this to see whether you were looking for a historical phrase, and I see that "regis voluntas suprema lex" is a common statement. In that case it means "the will of the king is the supreme law." Without "regis" in there, however, it becomes more ambiguous.
To make it less ambiguously "My will is the supreme law", I'd change the order to "Voluntas mea suprema lex"
Horae quidem cedunt et dies et menses et anni, nec praeteritum tempus umquam revertitur nec quid sequatur sciri potest. Quod cuique temporis ad vivendum datur, eo debet esse contentus. --Cicero, De Senectute