This works for me. It's a bit tedious, but it keeps my sims fine!
IF you have the Vampire pack installed:
1. After typing in testing cheats on, type in sims.get_sim_id_by_name
2. Turn your Sim into a vampire: traits.equip_trait trait_occultvampire
3. After that, you'll need your vampire to be a Grand Master: stats.set_stat rankedStatistic_Occult_VampireXP 1593
4. Then, you'll choose the dampened emotions perk (whichever strength you prefer)
5. After choosing weaknesses, you'll then need to get rid of them, because your Sim will still have these weaknesses after being turned back into a human (or so I think): bucks.lock_all_perks_for_bucks_type 40962 false
6. Last, turn your sim back into a human: traits.remove_trait trait_occultvampire — your Sim will still have the Dampened Emotions moodlet despite not being a vampire.
I hope this helps!
