Better to have normal mindset (in between winning and losing mindset) or the best word could be "for fun mindset". If you always use winning mindset but the fact says the opposite then it could be very bad for your emotional/feeling. You may feel very bad, regret it too much, etc. Think that winning and losing is part of gambling (although most of the time we'll lose), don't think one side only.
Being positive is indeed good but I can say that winning mindset can be called as over confident which is not good at all.