Christianity turned me into an angry and bitter person,
So has every other path you've taken, you are the common denominator between every faith you've practiced.
Before you stopped practicing Witchcraft you were into it for being able to do all kinds of retribution, this temper of yours is inherent to who you are rather than what life you've practiced thus far.
but unconditionally loving and forgiving other people and myself does not make me feel angry and bitter
You have extreme ups and downs, and you always externalize your room to appreciate things as if it were their fault rather than the fault of your own perceptions.
Your mood swings are a part of you, they will be something you will be contending with no matter what faith you prescribe to. With demons like yours I'd probably become a monk or something.
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