Aldeburgh :( Ohhhhhhh, yes! Feeling low and blue is totally normal even when your baby doesn't have reflux or allergies. When he or she does..... well.
I definitely experienced a very difficult time with DS, could not cope at all with him and DD as well, who was under 3 at the time. DD1 ended up in nursery 4 whole days per week (up from two previously) as I just couldn't cope.
I really feel for you. Having a reflux baby is a totally different kettle of fish to having a 'normal' colicky baby. Unless you've had one, it's not possible to have any idea how it really is, IME. Are you getting any time to yourself at all? It's so difficult, being pulled in all directions - needy, supplanted DC1, wailing, unsettled, non-sleeping DC2....
For me I am sure part of the difficulty was because I was confident it would be so much easier the second time around. That's what everyone kept telling me, that's what everyone else expected. It wasn't: Rather than beatifically breastfeeding my baby (come on, I'd done it for two years already) I was instead to be found mopping up sick and chasing a bottle-averse baby around for hours and hours a day on no sleep while also trying to look after DD1 who wasn't (and still isn't) undemanding any sense of the word. Big shock to the system. NO doubt you're having similar.
Are you getting any time at all for yourself? Still doing the night feeds or have you gone over to FF completely now? If you've stopped could your DP maybe do a night for you, and you retire to your DC1's room? spare room with some wax earplugs and a blindfold (can you tell how we survived our refluxy baby!?
Also, if you have given up the BF, then your hormones will be undergoing a major, major change and that might make you feel up and down. Don't underestimate the significance of this would be my advice :)
Congestion aside, how has she been this week? More screamy/ wriggly/ vomity, or less? More sleepy, or less? The weight gain is really good although I don't think it rules out CMPI.
It's good that your GP is still prepared to try the hydrolysed next week if things are not improving.
And about your DC1 - they are tough, you know, and can stand a fair amount of benign neglect in the wake of the new sibling. It's normal to feel sad and confused about your relationship with DC1 - your little dyad's no more: now you are three (and four when your DH is at home).
Mummyandmimi, certainly sounds like something is going on, and it's so, so infuriating to have that feeling like you're banging your head against a brick wall. Generally, mums do know best and it is so easy for docs to write off concerns as 'anxious mum'. Is there anyone else at the surgery you could see? Re-register somewhere with younger GPs who might have more of a clue?