((Hugs)) it's so hard when you feel worried about your supply and whether LO is getting enough, I'm sorry you had mastitis to contend with too. Is LO now growing well? What are nappies like, plenty of wet and soiled nappies? Is poop normal yellow/seedy?
As you probably know stress and worry can impact upon your supply so although I appreciate it's not an easy thing to do, trying to make sure you relax as much as possible, drink plenty of water, get a good diet and take some time for yourself will be really important. Supplementing, though it has its place, will impact negatively upon your supply and the best way to make more breast milk is to let LO have pretty much unrestricted access to the breast whenever they demand. I'm not sure how old your LO is but I'd try spending a good bit of time skin to skin, maybe taking a bath together, anything to encourage that feeding instinct and encourage LO back to the breast. Babies are much more efficient than pumps
Do you offer both sides per feed? You can also switch back and forth between sides more than once and offer the breast again 30 mins or so after a feed when the milk will be higher in the dense calorie fats. What about trying some breast compressions while LO is feeding too? LOs can develop a preference for the bottle as they don't have to work so hard to get the milk if flow is too fast, are you using the very lowest flow rate teats?
Hope some of those suggestions are helpful and I'm sure others will be along with some more thoughts before too long x