I hope you don’t mind me posting, as I have never had a professional declutterer in, but I have helped a few family members and friends with declutterring. And I am currently trying to go through a decluttering process myself. I suppose what that means is that it is much easier to do a decluttering process when you do not have the emotional attachments to the stuff (if it is someone else’s clutter) than if it is your own.
I’d say you need to try and have a very open mind before you go in, and be receptive to changing your mindset about the stuff you have, and why you keep it, and what you actually need. Unless you seriously change your mindset, then any improvements you make won’t last!
Key turning points for me have been the realisation that life improves in so many ways (for the whole family) when rooms are tidy and surfaces are clear. Everyone (in our house) is less stressed when there is no visual clutter, and keeping on top of things like dishwashing and laundry become easier (for whoever does them) when there is not always an ongoing battle of keeping surfaces clear and repatriating stuff (flotsam and jetsam) around the house.
Being able to let go of stuff is so hard - hopefully the declutterer will support you to work through why you hold onto things. eg birthday cards - I always used to think I had to keep them, but never actually read through them again, so may as well just recycle them after a week? There may be the occasional card you want to keep (from your children when small, eg, so that is fine to keep), but the rest is just ‘stuff’. And ‘stuff’ can be a massive burden.
For me, I have held onto stuff in the past as I have felt that I ought to, if someone has given me something, even if I don’t like it. But actually, I now think that the loved one who gave me the object that I no longer actually want or need would not want it to be a huge burden to me, and be making my house difficult to live in.
I have also found Dana K White (books & blogs) to be really helpful. The idea of the ‘container concept’ where you don’t just buy more storage containers, but actually stop and think - the shelf / cupboard for mugs that I have will only hold x mugs, so I will just choose my favourite x mugs and then will donate the rest. The same with bookshelves - use the existing book shelf as a natural limit to the amount of books you keep. And then operate a ‘one in, one out’ approach.
Good luck with the decluttering!