1. For one, it is extremely difficult to make my Sim happy. She may have a bunch of positive moodlets, such as great sleep, nicely decorated, clean clothing, having a blast, sometimes even all at once, and yet her mood is very red making her "miserable". I might have one or no negative moodlets at all but her mood still plummets. Now her job performance would not go up, she does not want to clean, and neither would she practice her skills.
Hopefully my first post will be a helpful one.
I have found, through t/e, that any hairs by Peggy have this effect of some of my sims. I have only used the ones converted by Anubis that I got from MTS, but every time I add 1 to my mods folder, at least 1 usually happy, fulfilled sim's mood plummets into red and nothing raises it. It doesn't effect every sim on the lot and I'm not enamored of any of those hairs to try and figured out why it is happening. I just pull them and all my sims go back to normal.
Check and see if you have any of these hair packages and get rid of them ASAP>