Jean-Paul Sartre is not often associated with positivity or hope. However, you may be surprised by the quote below which is from his play, Bariona. In an article looking at Sartre through the lens of positive psychology, Quackenbush, Lockwood and Cyr argue that Sartre's philosophy may be summed up as: I have dispositions towards future action, but this can be altered.
You are suffering, and yet your duty is to hope
This can provide the bridge to hope from what has happened in the past (and gone wrong) and what can happen in the future (and could go right). In order to allow our 'disposition' (our nature) to change we need to be ready to let go of what has happened and allow ourselves to accept that we may even surprise ourselves with what comes next.