Count, I am like you - all or nothing , and I am also a die-hard-breastfeeding-is-the-best-thing person too at heart. Well, I was with DD1. I fed her till she was 2. However, when I had my kind-of-breakdown when DS was about 5 weeks old, he went on to formula then and there as basically I needed a break from the constant feeding and crying, jiggling and carrying. IT was insane and I was at my complete wits' end. Even given all the issues we've had with him and formula since then, there really was no choice from my mental health standpoint, and looking back now, I still think it was the right thing to do. So if you do give the odd bottle of formula, really, do not worry. If you're already giving things like yoghurt to him then formula is no different at all.
Plus, your milk supply is really, really well established - if at some point in the future you decide to ditch the formula, then you should be able to do so. It can be challenging in some circumstances to re-build milk supply but usually not so difficult - and I say that with some confidence because with DD1 she was bottle fed entirely for the first 8 weeks of her life before I went through the relactation process, and she was back on BF only by 12 weeks. You could always use a sippy cup so he doesn't associate the bottle with comfort - you want him to still see boobs as a main source of both nourishment and comfort I guess. One bottle a day won't make a hill of beans to your supply unless it's precarious already, and I doubt it is with such a regular feeder!
I am so sorry that Andrea didn't work out for you. Did she really have no further suggestions for you 
?
I am also on antidepressants (currently two types - sertraline and mirtazapine) and they have been nothing short of miraculous every time I've taken them (I've had a few episodes, this is not my first). I hope they do the same for you. You can get some slightly odd effects in the first few weeks of taking them, but this wears off generally.
I really hope they help you feel better, but really, it's not that you're not coping at all, it's just that you're going mad with lack of sleep.
pugsmum oh dear, that sounds very difficult. When you say he is very windy, what are his symptoms? Does he just fart a lot? I ask because I don't think at 5 mos he should be having digestive discomfort that is causing him to be wakeful.
I know what you mean about discomfort and not wanting to do anything - but I wonder if it'd help if I told you about our recent experience with DS? Now, he has been a pretty unsettled little chap and very, very, very difficult to get to sleep as you describe. Never slept for more than 25 mins at a time in the day. He has reflux and cow's milk protein intolerance (both now treated) BUT since doing sleep training I have come to realise that lots of the sleep (or should that be non-sleep!) behaviours I attributed to pain (e.g., waking squirming, kicking and wriggling, rotating his head to one side in a very odd manner 
) were actually just....habit and his idiosyncratic way of making his preferences known, plain and simple. Only you can judge whether that might be true of your DS, of course.