How can i deal with pet euthanasia...? HELP!!!!!!

the vet is going to perform euthanasia on my dog tomorrow..ive been crying my heart out all day and i dont know what to do. i feel guilty that i am depriving him of his right to live but i feel bad and i think that if i keep him alive and in pain because of his mouth tumor..i really dont know how to deal with it i have no other choice because hes scheduled for euthanasia tomorrow....