Are you trying to go from every day old dose to every day new dose straight away? If so, some people are very sensitive to changes in dose and a slower approach is far easier to cope with.
https://patient.info/forums/discuss/pmr-gca-website-addresses-and-resources-35316
If you follow this link you will find a thread with a load of links in the first post - all reliable information and contacts - and in the replies part you will find a slow reduction plan together with an explanation of the whys and wherefores. It has been used successfully by many people on the 3 forums in the UK (who live all over the world) and has been seen and approved by various rheumies - and is currently in use in a clinical study in the north of England. It may appear slow - it isn't slow if it works!
You haven't told us how long it is since you were put on pred that I can see (I haven't time to read the entire 70+ replies in your thread just now, I have read them as they were posted), just that it is after 18 months of symptoms. However, the impression I get is that it isn't long - and you are already failing with a second attempt to reduce from 15mg to 12.5mg. You MUST be patient - yes, I know you hate the whole set-up, PMR is bad, pred is worse to most people, but you have a very basic choice: take pred, learn to accept it and work with it to get the best out of it, or go back to the pain and disability I'm sure you had for the last 18 months.
Top experts (in the "Bristol paper" in that link I gave you) keep their new PMR patients on 15mg for at least 6 weeks. Only then do they start to reduce. Yes, they say straight to 12.5mg, and it works fine for some people. However, other top experts have recommended in the past that in PMR no reduction should be more than 10% of the current dose - here, that would be not more than 1.5mg for this first time. By 10mg/day you are down to 1mg - so why not start with 1mg steps? If you feel you MUST do the 2.5mg, then use the dead slow and stop approach - it does work doing that for some people.
A lot depends on how active the underlying autoimmune disorder that causes the symptoms we call PMR is - if it is active you will find it more difficult to reduce far to start with. You have had this for a long time - there is probably a lot of stored up inflammation to clear out. Have you had your ESR (sed rate) and CRP (C-reactive protein) levels checked? Were they high to start with? If so, have they been checked now before this attempt to reduce? Were they already pretty much as low as they are likely to go? If they weren't low then you are setting yourself up for a fall at the first hurdle.
Almost everyone I know hates pred - I don't, it has given me my life back and I've been taking for 7 years - but most people come to an agreement with using it: it works to give a reasonable quality of life until the cause of the PMR burns out/goes into remission, whatever you want to call it. Which it does for the majority of people sooner or later, usually 2 to 6 years or so for 75% of patients. Then you will need no pred. Unless it returns - which it can. In the case of PMR there is no choice of medication, only pred reliably manages the symptoms. Some people find methotrexate helps to achieve a slightly lower dose - many don't - but it may be it isn't "just" PMR they have, late onset RA can appear identical but it usually responds to methotrexate.
The more you tell us, the more we can help with advice and suggestions - and there are plenty of people here who will offer their experiences in an attempt to help you.