sorry to hear you've been having a tough time of it Ayla123, but
& welcome to the group anyway! It's always difficult when baby is struggling to gain weight, regardless of whether your BFing or FFing (there's a mix of both on here), but it sounds like you're doing what you can, so try not to beat yourself up about it.
Other than the slow weight gain, is your baby having plenty of wet/dirty nappies? Does she seem alert when not asleep or feeding? Does she bring any milk up after a feed? Does she struggle with wind at all? Is her soft spot sunken in at all (sign of dehydration n not getting enough)? If all is well there, then despite the slow gaining, the chances are your daughter is getting enough.
Is she gaining at each weigh-in rather than losing? The important thing is that even if she only gains 1oz, that is still a gain, rather than her losing weight!
How much does she take from the bottle at each feed? My son (7mths old) still wont drink more than 5oz at a time from a bottle & takes in so much air he's quite uncomfortable afterwards & cries until he gets the wind up, which is one of the main reasons I kept BFing & stopped supplenting. It's only in the last 6 weeks that his weigh gain has been good, but before that he was swinging between the 0.4th-2nd centiles, but when I took him to the Drs, they said he was absolutely fine.
If you're still worried about growth, get your Dr or HV to measure your daughter's length & head circumference, as apparently these are much better indicators of whether a baby is growing sufficiently than weight alone.
With the BFing, if you did want to start again, check out the BFing section- a few of the mums on there have done relactation & successfully gotten their babies back on the breast, so it definitely can be done if that was the choice you wanted to make!
Sorry for all the Qs