Negativity and Positivity can BOTH screw you over, just in different ways. If you are too negative, you will intimidate yourself out of trying things, get too hung up on past failures, and won’t be much fun to be around.
If you are too positive, you will be overconfident, fail to anticipate how your plans can go awry, and constantly let yourself off the hook for your failures, without learning from them.
Better to be hopeful, but also objective and realistic. Anticipate how things can go wrong, and make contingency plans. Analyse your failures and learn from them, without beating yourself up.