Is merely seeking sincere repentance and forgiveness from Allah sufficient for a new beginning, or is expiation necessary for the days I missed fasting in Ramadan without making them up, for the abortion, and for the usurious loan?

You must make up for the missed days of Ramadan (definition) fasting, along with paying a fidyah (definition) (expiation) for each day whose make-up was delayed until the next Ramadan began, unless you were ignorant of the impermissibility of the delay. As for the abortion of the formed fetus that you initiated, it necessitates that you pay the diyah (blood money) to its heirs and perform sincere repentance. It is also recommended to fast two months as expiation.