As Christianity suggests, is loving everyone a sustainable lifestyle? Can you always forgive and have no hatred?

Christianity and its love-centered worldview really interest me. However, when someone is cruel to me I often cannot be forgiving. It feels like being naive and I want to respond in kind. Are Christians able to overcome this and can they really love everyone? Is it something that our selfish human nature can accept? And don't you think that it leads you to be weaker and to fail at competition based situations since you will always help your opponent?

As Christianity suggests, is loving everyone a sustainable lifestyle? Can you always forgive and have no hatred?
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