Why should a particular deity or form be worshipped if all are said to be God?

Question: Why should a particular deity or form be worshipped when it is said that:”you are god…I am god…”

The statement you have quoted above is not found in any authoritative scriptural text such as Bhagavad Gita. In the Gita the Lord clearly states that He is the cause of all causes and the source of all that exists:

aham sarvasya prabhavo
mattah sarvam pravartate
iti matva bhajante mam
budha bhava-samanvitah

“I am the source of all spiritual and material worlds. Everything emanates from Me. The wise who know this perfectly engage in My devotional service and worship Me with all their hearts.”

Elsewhere, He states that He is situated beyond both the fallible and the infallible, and thus He is the Supreme Person (purushottama).

yasmat ksharam atito ‘ham
aksharad api cottamah
ato ‘smi loke vede ca
prathitah purushottamah

“Because I am transcendental, beyond both the fallible and the infallible, and because I am the greatest, I am celebrated both in the world and in the Vedas as that Supreme Person.”

The living entities are not God, but His eternal fragmental parts and parcels:

mamaivamsho jiva-loke
jiva-bhutah sanatanah
manah-shashthanindriyani
prakriti-sthani karshati

“The living entities in this conditioned world are My eternal, fragmental parts. Due to conditioned life, they are struggling very hard with the six senses, which include the mind.”

For this reason, Lord Krishna instructs us in the Gita that we must surrender to Him and take Him as our “only shelter”:

sarva-dharman parityajya
mam ekam sharanam vraja
aham tvam sarva-papebhyo
mokshayishyami ma shucah

“Abandon all varieties of religion and just surrender unto Me. I shall deliver you from all sinful reaction. Do not fear.”

The words “mam ekam sharanam vraja” mean that we should accept Lord Krishna as our only shelter. This is the conclusion of Bhagavad Gita.