I think "did" work as a normal verb in this situation, but I'm not sure about it. I would like to have your opinion. Thank you!
To do one's best is an idiom which means to do something as well as one can, so the sentence may also read:
I did everything I could.
I did my best.
In both examples the verb to do has its literal meaning of to carry out an action/actions, so did, in the examples, is just the past form of the verb.
Helping verbs are verbs that help the main verb in a sentence by extending their meaning. There are two types of verbs that can be used as helping verbs.
- Auxiliary Verbs
- Modal Verbs
These verbs mainly used to express tense or add emphasis.
I am having a cup of tea.
She is writing a letter.
You did not do the best you could!
Here as you can see am, is & did are helping verbs to help the main verb to represent the tense of the sentence.
Modal verbs further modify the action or meaning of the main verb.
- She can sing that song.
- You should learn English.
Here can and should are modal verbs.
So in your question, as you can see did does not help any verb to express the tense in that sentence. did is the only verb in your sentence. So, it is indeed the main verb, not a helping verb.
I hope this will help you.