thank you for seeking help,
A tear on the foreskin after sex is very painful, you need to keep the area dry with complete hygienic condition, apply some local antiseptic, maintain abstinence for at least 10 days or till the tear heals.
A painful sore (ulcer) on the head of the penis or on the foreskin is often due to a sexually transmitted infection and often occurs together with urethritis
. in this case you need to show to a doctor as soon as possible to give you a diagnosis as well as treatment for the lesions.