Is my prayer valid, or must I repeat it, after I felt some wetness then became certain it was pre-seminal fluid that exited before the prayer?

So long as you were not certain of the emission of madhiy (prostatic fluid) during prayer, and you considered the possibility of its emission after the prayer, then your prayer is valid and you are not required to repeat it. This is because the default assumption is that you were in a state of ritual purity, and this default assumption is not negated by doubt, and ghalabat al-dhann (preponderant probability) falls under doubt. The jurists have stipulated that it is mustahabb (recommended) to repeat the prayer for one who doubts the emission of something during it.