WTF ?
DO NOT BREAK THE LOOP ANYWHERE !!!
THAT'S BRUTE FORCE
If u r using typical topologies (error amp, pass transitor, feedback net), u must break the loop in the feedback path (which connects with the gate of one of the differential transistors), and introduce a large inductor in series (e.g. 10 H). Then, u should place ur ac signal with a huge decoupling capacitor (e.g. 10F) in series, an introduce it into the feedback gate. then run ur ac simulation. Let me now if u need further assistance.
ez, don't u think ?