Hey sprite, I was there right where you are when my dd was the exact same age - not enough dirty nappies, the scales showing weight loss, and the most helpful breastfeeding-friendly HV saying it was time to give formula. Devastation. I bawled. If anyone had told me then I'd have gone on to feed dd for 13 months I wouldn't have believed them. I thought it was the End.
Mixed feeding really isn't the end, though! Mixed feeding is a great friend to breastfeeding if continuing breastfeeding is what you want. For me it was 24 hours of formula in those early days just to flush through some poos. Whatever you end up doing, once the poos are coming thick and fast you'll feel much better, I bet you anything.
I hope this doesn't sound too much like laying it on in the favour of continuing to breastfeed all of the above is just trying to reiterate that bf is HARD, that nobody tells you quite how hard it is and - as Mrs DV has said how different it is from ff. Equally, it's difficult to convey just how much easier it gets because when you're so tired, sore and worried, somebody talking about even the near future seems besides the point.
I always say to DP - I am so glad we'll never have to go through the 'first baby phase' again - it really is a bomb going off in your life, but hang in there, it will all pass so soon.