I'm sorry to hear that the doctor visits are undermining your confidence. She should not be bashing the quality of your breastmilk or discouraging you from using your expressed milk for supplemental feeds!
One thing to remember about weight checks is that each one is subject to a variety of uncontrollable factors that may slightly impact the accuracy of that one reading. The best way to know that a baby is growing well is to look at the entire history of weight checks over several weeks, not just one number. And, of course, to look at the baby, to evaluate her behavior and developmental progress.
I would also caution against any simple measurement to evaluate the quality or nutritional content of your breastmilk. The fat content of breastmilk in particular varies substantially over the course of the day for many mothers -- if the one sample sent to the lab happens to be mostly foremilk, you could be told inaccurately that your milk is "too thin." A mother has to be seriously and chronically malnourished before the basic make-up of her breastmilk would be altered -- and even then, the problem would simply be too little milk, not "poor quality" milk.
I think you are wise to look for another doctor who is better educated and more supportive about breastfeeding. In the meantime, there may be a clinic or an IBCLC nearby who could help you do weight checks and, more importantly, help you evaluate the results of those weight checks. My one caution about using a home scale is to be wary of letting the scale come to define your entire breastfeeding relationship. Weight checks are just snapshots that reflect one aspect of a complex reality.
Glad to hear you are hanging in there, being proactive and determined. Keep up the good work, and keep us posted.