Baby isn’t gaining

My baby has been exclusively breastfeed since birth but still at 4 months she has yet to gain enough weight. We are have to supplement with formula now —cue all my tears and feelings of failure.

She latches well, nurses well (though not aggressively) and rarely ever spits up. So basically the problem is with me - cue more tears.

My firstborn had the opposite problem which makes me ask why. Why is my milk suddenly not enough? Or is she really not pulling out the hind milk that holds the calories? I don’t know... my pediatrician can’t speak to it.

I plan on reaching out to a local lactation specialist/group. However I’m hoping you ladies can speak encouragement to me and help me through this emotional pit I’m probably digging myself.