Am I able to apply for a certificate of guardianship over the estate on my own without a lawyer?

You can file anything on your own.

The question is can you do it correctly?

Do you want to file a Petition for Guardianship?  Massachusetts does have websites that will provide the forms necessary to file and could be helpful in doing same.  I will include the website here but I can not attest to the accuracy of the forms or the validity of the information contained in the website.  Often the Court clerks can guide you but can not give you any legal advice.  Only an attorney can do so.

Obtaining guardianship can get complicated.  It may be advisable to seek legal consultation.  Good luck.