The first three months are not the pattern

Starting work costs money before it pays any. A deposit and first month up front, furnishing a room from nothing, a travel pass, clothes for a workplace with a dress code, and the ordinary gap between starting and the first pay date, which can be six weeks.

Budget the first quarter separately from the rest of the year, and go in with a buffer that covers it. Graduates who borrow at high interest in month one are usually not overspending; they simply modelled month one as though it were month nine.