There's really no way to forgive or pardon oneself for previous mistakes or slip up like you referred to. Other than to understand this concept. "There's no reason to beat ourselves up when we can't handle unforseen circumstances that are beyond our capability" sometimes the things we call slip ups or mistakes are circumstances that we cannot handle and that's the main reason we have to understand that we're only human.
The truth is that whether we like it or not slips ups and mistakes are part of life and part of our existence as human being, the thing is that when we experience things that we seem we can't really recover from we need to understand that we aren't the only people going through these issue in our lives and if we decide not to move forward then we remain stagnant and when we remain stagnant we hardly grow or develop and when we don't we hardly exist.
So basically beating ourselves up for our mistakes doesn't work as I said in the first paragraph, accepting or understanding that our circumstances in life sometimes is beyond our capacity and the fact that we're only human and we're prone to mistake is the reason why we should find that motivation to rise again and push forward in life