The line is poetic. "Paghimig", "lulan", and "pinagtatanto" are words we ordinarily don't use anymore. I would interpret it as: "The melodic idea in my head that I am trying make sense of."

Just curious, where did you get this line? The words don't seem to fit well. Either due to error in typing (typo), grammatical error, or maybe poetic license? "Himig" --> tune, sound, music, melody "Lulan" --> carry, load or ride (as in a bus, boat, or any vehicle); this is usually followed by the object that is carrying the load (e.g., lulan ng sasakyan--> carried by/ loaded in/ riding a vehicle) "Tanto" --> realize, understand "Ang paghimig na lulan" --> The melody that is brought/ carried... "na aking pinagtatanto" --> ... that I am trying to understand. I hope this enlightens you a bit. Taken out of context, the statement might not make much sense.