My dear Min,
Of course you are allowed to get angry. Its an emotion, just like fear,sadness, laughter and love. It means you are human. I have got angry too and have regretted each time that I did, so much. I have been caring (as best as I can) for the past four and a half years and, just like my long suffering but beautiful wife, I get my good and bad days. Dont feel too upset with yourself after throwing a tantrum, just have a short break, or go to the shops, then when you return you will feel a little better. The anger will pass and the situation will slowly return to normal. "Be cheerful. Strive to be happy".
Love
John