I would like to just agree with many things people have already said. DD now 19months, breastfed fine, but put on little weight, she maintained a steady gain, but she was small to start with so this gain was not much, then it dropped a little. I was pressured contuniously by the healthvisitor to stop, or to top up more, mix feed, but DD refused, thankfully.
I continued to exclusively BF until 5.5months, weaned and BF until she was 9 months then moved on to cows milk, her own choice. She was always active, met her milestones early, weed and poo'd fine, and I am so glad she refused a bottle. She is still small, but thats how she is going to be, her weight and height are smilar in charts, as they always have been, and she is a perfectly healthy girl.
The thing that kept me going was once I saw a stand in health visitor, who reminded me the chart I had was a bottle feeding chart, so although she was low on it, she would not be if it was correct chart. my other health visitor conveniently could not find a breastfeeding chart for me!!!
It sounds to me like you are doing a great great job, and have perservered through some obstacles. sounds all good to me,