Church or Synagogue. If you have a relationship with a pastor or rabbi, don. For many people, religion is a framework of life. Seeking professional help is absolutely all right and very common. Grief and depression are just as real over the loss of a pet as they are over the loss of a person.
Some professionals offer pet loss support groups.
At a group like this you will be with other people in the same situation as you who understand your grief and can share your experiences. Friends and family. Many of them have been with you in your grief from the time of decision or the receipt of the terrible news. And most have known your pet as long as you have. It may be difficult to accept help, but if someone offers, think about accepting it.