Azmaʿti is a second-person feminine singular perfect verb from the root z-m-ʿ, conveying a firm resolution. Qad with the perfect reinforces that the decision has already been made within the hypothetical situation.
Ṣarm fundamentally denotes cutting or severing; here it is the ending of the emotional relationship. The first-person suffix -ī marks the speaker as the one from whom separation would occur.
Wa-in kunti qad azmaʿti ṣarmī presents a condition, not an assertion that separation has certainly happened. Its force is “if you have decided to leave me”.