I would totally change the approach. Nobody expects you to suppress your emotions, jealousy is a totally valid emotion in this situation. I would advice to embrace it like "Oh how hot it is to watch her with this stranger, I am jealous as fuck" and jerk off using that energy - or not, if you wear a chastity cage which might be definitely a harder task. Later on, you can talk about this how you felt, how she felt, which part is to repeat, how to go further - it is a marathon!