What Should I Do If My Penis Is Not Able To Enter Vagina?

i am 28 male and i married before 4 years and my wife is 24 i have a son. before when i have sex my peins was going inside of vergine but after my son born she has stiching of virgina after that my penis is not entering her virgina i used lubercation also and try to put oil its very pain full she answers me kindly suggest how my penis will enter her virgina and annother thing comparing to other i am little fat and my peins size is 5 inches but very fat plz plz help me to solve the problem i would be kind full for u sir,