One of my sims is constantly frustrated, constantly tense, very emotional...very annoying. How to fix or delete moodlets (or whatever they are called)? I come from TS2 and i never had to deal with this personality thing, Is there a cheat to just make them neutral so i can play with them however i want without being interrupted because they are feeling a certain amount of stress? He is crying constantly i really need help because i just turn off my game because i get stressed just playing =[

I wish there a cheat for this in real life, because that description sounds pretty realistic

Seriously though, all of the moodlets have causes that relate to the same things that affect Sims in TS2 (their needs and whether they are doing things they want or fear). It may just take a little while for you to learn how to most easily keep your sims satisfied while they go about their lives. If you hover over their moodlets, there should be a description of what they're feeling and in brackets () should say what the source of the mood is.

For a short term solution, entering testingcheats true in the cheat console will allow you to shift-click sims and select 'Make Happy' to fill their needs and reset their mood to happy.
It's hard to have a long term solution without knowing what is causing this sim so much stress- take your time with the learning curve, as TS4 does tend to be the easiest of the four games. Also, if you're playing with a lot of expansion packs it may help to disable them while you get used to the game- just so you aren't having to balance things like sentiments, lifestyles, mood swings, so on if you aren't too familiar with the base game yet.

Here's also a tip that might help (although probably not very much): What traits do your sims have? Traits also have (questionable) effect on moods (for example, a cheerful trait almost always eliminates most stress a sim may develop in situations.) Happy -ahem- Simming.