You speak in terms of extremes. Perhaps Your *sweetness* is partially a put on. (and I don't mean this in a bad way hear me out)
by put on I mean perhaps you keep the sweetness role in place when what you are really feeling is anger that builds until it reaches an extreme. Maybe you need to learn how to live in the balance between these two extremes and then you won't have such mood swings.
I would suggest you try to catch yourself *putting on the sweet face* when you are just starting to get annoyed with someone, but don't feel ready to speak out.
Then practice speaking out to assert yourself before this annoyance builds to anger ....and things explode.
Diplomacy is a lost art. You should read books about assertiveness training too just to get some tips on how to do this.
Sometimes when someone says something that *piques* your annoyance reflex say something like "Oh, be nice" or
"Come on now behave"...just little pressure relief valves to let your true feelings out so the pressure doesn't build to these extremes.
I think if you can do this you will find you spend more time being the *sweet angel*. Because that's probably who you really are inside.