I nickled-and-dimed a client on change requests, alienating that client and making myself appear less professional.
...
I should have just sucked it up and done the work, leaving both of us with warm fuzzes in the end, even if I took a slight loss on the contract.
It's interesting the OP lists this as a mistake. I'd say he made the right move, as some clients will "just one more thing" you to death. Once the changes go beyond the agreed-upon scope, you should start charging. Alternatively, adjust your estimate to take into account client changes.
Said client did not have a whole lot of money, and while the initial contract amount was commensurately very low, she really didn’t appreciate me charging additional for some very minor changes.
This was the real mistake.