It does sound like an MS diagnosis is a bit of a certainty
If MS is anything, it is unpredictable. However, that doesn't mean that we shouldn't do a bit of planning as well as do whatever we can to stay as healthy as possible; to fight it. Fighting it is a bit harder with PPMS because there are no DMDs at the moment (there are some trials going on though, so there may be something relatively soon), but there are still things you can do.
Something that some PPMSers swear by is LDN (google ldnresearchtrust for info). It doesn't work for everyone, but it has got to be worth a try. There are one or two people who use the EL pages who are on LDN with the blessing of their neuro, but most do not approve because LDN has not been clinically trialled. So if you mention it to your neuro, expect a scathing reply! For what it's worth, if I had PPMS, I would be on it in a shot. It might not work, but at least I'd tried.
Other things that are definitely worth doing are improving your diet (you've ticked that box already), exercising and keeping as fit as possible (tick), keeping as flexible as possible (if you aren't already, please add regular stretching to combat spasticity), taking vitamin D3 supplements (if you haven't had your levels checked, please do; in fact, get everything checked!), omega 3 is supposed to be good for MS too (get one with a decent amount of the stuff in it that's good for the nervous system, sorry, can't remember what it is, but most omega 3 brands are low in it).
Another thing you could try is HBOT (hyperbaric oxygen treatment). This is available at most MS Therapy Centres. There is no evidence that it slows progression, but at least one small trial found that it helped with spasticity. There is also some (mixed) evidence that it can help with bladder problems. Quite a lot of people report that it helps with fatigue too, although this hasn't been found in the few studies that have been done. Worth a try though! The MS Therapy Centres also offer physio, exercise classes, reflexology and things like that, so are worth checking out anyway, even if you don't fancy HBOT.
In the meantime, it is really important that you get the help and support that you need to minimise the effects of your current symptoms. The neuro will be able to recommend meds or therapy for symptoms that you aren't coping with. You also need to get an OT assessment and see a neurophysio if you haven't already. With MS, you are covered by The Equality Act, so your practice needs to make "reasonable adjustments" to allow you to stay in work. So perhaps a change in hours or different equipment or aids around the office? On a completely different front, you need to check your paperwork for any possible critical health policy you might have - it might make the world of difference to your financial planning. It would also be sensible to check out your pension - exactly what are the terms and exactly what will you get and when?
A diagnosis is scary. No question about it. However, as you learn to live with it and with your MS, you will find that, actually, you can cope. Things that you are terrified of at the beginning either don't happen or, if they do happen, they aren't as bad as you feared. Yes, sometimes MS can be quite overwhelming, but we manage and we carry on. It is only a very small percentage of MSers (including PPMSers) who aren't able to. For the rest of us, life can still be, and is, good.
Please make sure that you are prepared for your appointment - get what you need from it, e.g. recommended meds & therapy referrals. Make sure that you get your MS nurse's contact details (ideally an email address as well as a phone number). You could ask always about any drug trials for PPMSers too. And anything else you want of course!
I hope it goes well!