My father passed away and I'm a dependent over 21. Does the DD-137 form have to be done every four years?

You cannot be both dependent and sponsor.  You cannot extend your benefits to anyone else.  

Yes you will need to reestablish eligible status.  I thought it was every 5 years but that may have changed.  

Depends on the branch of service to which form is required.  If he was your sponsor then the surviving parent would need to do it 

https://branchta.org/what-is-secondary-dependency-...

The fact you are calling your parents shameful for not doing it before for 25 years is so wrong because guess what it was NOT always an option. You can’t be your own sponsor and have other dependents under you. This is something that you should have looked into before especially since your parents were older and you are in your 40s. Look at getting a caseworker, which is also something you should have had before 

You also drive a car and run up credit card debt but expect everything to be handed to you. Heck you even talk about getting married but want to keep social security. 

https://answers.yahoo.com/question/index?qid=20201...

You can do some work, even a part time job. 

The DD-137-5 form I mean.