Quitting breastfeeding

I dont know if I'm here to vent or for advice. I went to a lactation consultant today because my 8 week old is having trouble latching. Turns out she has a lip tie and is only getting about a half an oz a feed.. I found out she has only gained 3 oz in 3 weeks. My heart is broken and I feel SO BAD. I had no idea. She is so happy and doesnt seem hungry after her feeds and now I feel like i have starved and failed her. So basically i will have to pump now to ensure she is getting what she needs because she isnt latching efficiently. But i think i might just switch to formula.. I'm so sad because my whole pregnancy my goal was to breastfeed but I have a 4 year old and it is so hard to be attached to a pump all day. I feel selfish wanting to take the easy way out though.. ugh.