How Can I Win A Fight Against Someone Smaller BUT WAYYY Stronger?

I am 18 5'7" 140 pounds and never won a fight against a kid over 14 or over 103 pounds, I have never fought a kid over 15 or over 108 pounds, So far I have a record of 47 wins and 235 losses. Lost a fight to a 13 year old 5'4" 100 pound kid Tuesday night.