I'm not surprised you are seriously tired!! Have you spoken to a lactation consultant to help you with your milk supply? That may help with that aspect.
It's important that you know that this isn't your fault. It can be really hard to care for a reflux bub, and while it can cause you to lose confidence in yourself, it is still important to trust your instincts and do what feels right (no matter how much advice people offer).
How are the feeds going? Is he feeding well, or is that perhaps part of the problem? Reflux babies often have feeding difficulties, with feeding refusal being quite common. That, combined with stress (and exhaustion), does nothing for your supply.
There are a few things that I can suggest that may help, but it is up to you to decide what you would like to do, of course. One is that perhaps his reflux is still not well managed, so letting your doctor know you are still having difficulties might help. There is a wide range of medications and dosages that are used, so it might be helpful to consider that.
Another is, if you think it's possible his feeding has worsened, contributing to your lessening supply....since he has been on the Losec suspension for 4 weeks, are you using a new bottle, or is the bottle old? If the bottle is old, that may be part of the problem as some parents report them being less effective towards the end of the month. Sometimes making sure they are shaken well before every use helps. Also, their shelf life is sometimes quite short, depending on the suspension agent- as little as 10 days with some, so it may be worth checking what they have used, and what the shelf life is.
Something else that you may like to consider, is that it sounds like cows milk protein intolerant/allergy is a possibility, as it commonly is for reflux bubs (a medical article I read quoted the figure at 40%). If the losec suspension contains whey protein, that may be causing some problems (if he is sensitive). Since you are breastfeeding, if you did want to continue doing so, you may be able to look into altering your own diet, which may help you find out if food sensitivities might be an issue? If he is dairy sensitive, it may be necessary for you to avoid all traces of dairy, even by looking on food labels. That isn't easy, and if you do it for any length of time, it is important to get advice from a dietitian experienced in food sensitivities. Some bubs who are sensitive to dairy are also sensitive to soy, but others, like your daughter, seem to cope just fine.
If you have made the decision to start bottle feeding, then I would advise talking to your doctor or child nurse who could help with what formula might be best. There are so many to choose from. It may help to know that thickened formulas can be really effective for some babies, but not for others- for some it can make them more unsettled, so perhaps that factors into your thoughts. I don't think there's any magic answer, unfortunately. Sometimes it can be down to trial and error until you do find something that suits.
I hope that information helps you to figure out what to do
Reflux Infant Support Association
Note: This information is general in nature and should not replace the advice or care of your doctor.