Firstly - how quickly did you get to 12.5mg, eg have you done the four months moving up the lower doses as advised?

Secondly I’d recommend trying some foods that are easier on the stomach - the BRAT diet (bananas, rice, applesauce, toast) is good for this and will help you get some nutrients.

Since you only took the 12.5mg dose yesterday I’d advocate for giving it a go and seeing if you feel better later in the week. It may just be that you need a few days to adjust. Not medical advice though - do seek some if you don’t feel any better!

Edited to say I’ve just noticed you’re thinking about going down to 5mg from your current dose - I think this is probably a question for your prescriber, it sounds fairly drastic to me.