I'm so sorry it hasn't worked out the way you envisaged. Do allow your self time to grieve this if you feel like you need to. You can still talk to a bfing counsellor about stopping (hence the counsellor bit!) so don't be afraid to call one of the helplines or have a visit.
Good resources for formula feeding :-WHO guide to making up formula feeds,
guide on how to bottle feed responsively (I'm sure you are already but it's a good one as it has lots of tips that you may not have heard (or a quick refresher
),
report on infant milks in the UK
Hopefully they will answer your questions but in brief:-
Are there any good websites/resources? - Unfortunately there isn't much out there on formula milks that isn't manufacturer based (I've had a look on the formula brands forums and they are fairly frightening esp. after MN).
Are some brands better than others? - No, they are all much of a muchness (content is regulated except for the more recent additions like fish oils etc) so all will provide the standard amount of nutrition, it's just a case of trying a brand and seeing if baby likes it, if not then try another. Just keep to stage 1 (newborn to 12m) as stage 2 is difficult to digest (deliberately so to ward off early solids).
How much do they need? Try starting with 3oz and seeing if that is enough, if not make up 4oz and so on. You will probably need around 30oz a day though some babies will take less and some more. If baby is taking excessive amounts like 9oz a go or 50oz a day then try offering a dummy, or switching down a teat size. You probably won't notice too much difference between bottle feeding EBM and formula TBH.
Do you still feed on demand? Yes, it's now recommended to ff on demand. Just follow the same cues as before.
Can you keep made up bottles in the fridge? - Yes but freshly made is the recommended (and safest) way. If you do decide to make up bottles in advance, the important bits are - >70C water in bottles, add powder, chill quickly (bowl of ice cold water), then refridgerate for no more than 24hrs (the shorter time the better. Don't add powder to cold boild water except in an emergency it's the least safe prep method (barring doing that than storing on a window sill in summer!).
Cartons and disposable sterile bottles are good for having in your change bag in case of an emergency or hold up in getting home.
How do you know you're not overfeeding? - The how to bottle feed guide has some good info on what to look for (starfish hands/splayed toes being one sign of being overwhelmed).
In answer to your final point - you have done so much for your baby, and there comes a time when all of us have to stop (in everything in life not just bfing).
Please, please be kind to yourself and get as much help as you can with the PND, don't let others take over your son as it's so tempting for others to 'help' by taking a baby from a mum with PND so she can rest but if it happens to often it's not good for mum's recovery as she can feel like she's not good enough (it's not good for baby either to be looked after by a succession of people rather than a few loved ones like mum, dad and GPs), let others who want to help do everything but look after your son (nothing wrong with you having a break though).
Nothing like taking a break from housework to have a walk in the sunshine with baby in the pram, to come home to a clean house courtesy of a kind friend.
I've had PND, and got out the other side. We are legion, I tell you, you'll be surprised just how many mums have had PND, PTSD, PNP.....you are not alone (hugs) and some 