The analyses of a sentence #1

The narrator recalls his childhood.
He has lived with his mother, Peggotty the only maid of his house, his stern stepfather Mr. Murdstone, and Mr. Mudstone's eccentric elder sister in his late father's house in Blunderstone.
He was going to a boarding school near London on a cart, now arrived the way point Yarmouth.
He had lunch at the restaurant and now is about to resume his traveling on a new cart.

....................
I felt it rather hard, I must own, to be made, without deserving it, the subject of jokes between the coachman and guard as to the coach drawing heavy behind, on account of my sitting there, and as to the greater expediency of my traveling by waggon.
[David Copperfield by Charles Dickens]
I'd like to know if the comma before "I must own" plays the role of "that."
I think "to be made" is an object of "must own," so I was wondering why there is the comma before "to be made."
And I'd like to know why a noun is omitted after "heavy."
Thank you in advance for your help.