depends on ritsu's definition of abandon /j

generally it was a 1-time thing! it refers to rei's first year spent abroad. it's largely understood that this happened during rei's second year, where he spend largely his time abroad both dealing with family matters and searching for a treatment for ritsu's condition, his extended time away is what resulting in him being held back. although seeking a treatment was something done out of love for ritsu, it left him feeling immensely abandoned and hurt.

especially since what ritsu sought out wasn't a cure for his affliction, but just wanted to be able to get to be with his older brother... yet rei instead left out of his own "selfishness". of course, rei was also absent because of family matters, but ritsu has always felt abandoned when rei was busy with those matters as well.

ritsu regularly would feel this way growing up, even though being so absent wasn't out of rei's choosing. with their parents busy elsewhere, rei was essentially assigned adult duties since a very young age which would require a LOT of work regarding his family and their properties. he typically would be out of the house or elsewhere, and this left ritsu feeling regularly neglected throughout their childhood, and he would regularly lash out at rei for abandoning him. but, it i believe it was that incident that was the "final straw" for ritsu.