Lots of good advice here.
I would reiterate, try to find a local support group. Or phone a phone helpline. I rang the NCT helpline and got some very good advice.
All this stress is probably not helping. If you go to mixed feeding, then you it will probably help you relax about it all and that will help a lot.
I know you want to do what's best for your baby, but in the end, a stressed out mother is not best, so don't get guilty about FFing!
FWIW, I EBF my two DDs, DD1 dropped centiles and worried us all, but after getting her checked out by doctors and finding nothing wrong, I got some advice about making sure she was getting enough milk, which helped. She settled around the 9th centile and has remained there since (she is now 3).
DD2 was a much better feeder, but I don't know how much of that was because I'd done it before, so was more relaxed. Even so, she dropped a few centiles too, and ended up settling on the 50th.
I think birth weight doesn't have a lot to do with the baby's final weight, but it's naturally more worrying if they drop than if they pile on the ounces.
Good luck, whatever you choose to do.