People telling you to just 'throw stuff away isn't helpful. Buffybee has the best idea but at the end of the day it's you that has to do this.
Whether you choose to label yourself as a collector, hoarder, acquirer or whatever, ending up with the amount of 'things' you're indicating you have, usually has a trigger, which in your case appears to be connected with your ex. Its not as simple as 'getting rid of stuff', you need support to help deal with the root cause.
You need to acknowledge and be proud of the fact that you've made the first step by telling someone (us) about the problem, that is the biggest and hardest milestone to overcome in these situations.
I was in a similar situation and as much as I wanted to sort my home out, I had two things working against me.... my physical disability and the 'knowing exactly where to start'. I eventually reached out and was put in touch with a charity in my area that helps with the psychological side of these problems. I went to a meeting and said that I knew what the causes are and that I was in the right mindset to start clearing but I was stuck because of my physical issues. The group has been a great help since I joined them last January and they even arranged some physical help for me. We were going great until we hit the November lockdown and I've plateaued, I've not been able to do any more clearance work since then but I can proudly say that I've not slipped backwards. Once we're able to have people in our homes again we'll be picking up where we left off and I should be in a position to start getting some decorating done sometime around the summer.
Which 2 rooms do you want to use the most? Start there, anywhere there, I thought I'd need to have a rubbish bag, and 3 boxes (keep, sell, donate) but it's easier to start with just one box and the rubbish bag, anything that's not going straight into the bin goes into the box and can be gone through later. I like to call it 'reclaiming space' not 'decluttering' or 'throwing stuff out'.
Some would say that you're handling stuff twice, yes, but if you start in one small area and have the rubbish bag on one side of you and the box on the other, work through that area for 20 or 30mins (setting a timer for yourself), you'll be amazed at how much you'll have moved in that short time. If you feel up to doing a little bit longer, reset the alarm for another 10-15mins but don't overdo it or just keep ploughing through until you drop. Alternate between your 2 chosen rooms so that you can see space reappearing in both. The clutter wasn't acquired overnight, so you can't expect to be able to deal with it in a short space of time, this is a marathon not a sprint!
It can become overwhelming, you'll find things which have sentimental
or monetary value which may have become damaged over the years which may make you feel as though you don't want to carry on. Take a mini break, put the item to one side until later, maybe move away from that spot, go and finish your time session in your other chosen room to take your mind off the item that's just caused the mental 'wobble'. Start your next session by going back to that item, by then you should have got your head around the fact that it's broken/ruined, put it in the rubbish and move onto the next thing.
At the end of each session, even if it is only 20mins, reward yourself, sit down with a cuppa, watch a favourite TV programme, read a chapter of a book or do a puzzle, just something small that takes you away from the clearing. Each time you do this you'll get into the mindset of 'if I do 20mins of clearing I can have/do something nice'.
Now, coming back to the double handling of the things you put into the box(es), once you've cleared a big enough space in your rooms and got rid of all the rubbish, you can then go back through the boxes and start sorting into 'keep', 'sell' and 'donate' boxes, you might even find that you can bin some more stuff as you go through them, it's amazing how many duplicates of things you'll find. The optimum target is to say that including rubbish, you want to be aiming to remove (at least) 60-70% of the clutter from your home. It seems daunting thinking about such a large amount right at the beginning but most people find that once they get started and begin to see space reappearing, it gives them the motivation to keep going. Bear in mind that the 60-70% target isn't all rubbish, it'll mainly be things that you end up donating or selling.
I've nowhere near completed my 'space reclamation' as yet but I have been putting things on ebay over the past year and I've sold loads of stuff, which has helped me to pay off a credit card and to buy myself a new bed/mattress and a few other things I've wanted.
Once you've started reclaiming your space, it is important to minimise the acquiring of more stuff to take the place of all that you've removed from your home, I'm not saying you CAN'T buy new things but to be more mindful of whether you NEED them.
Help for these issues is out there, it can be difficult to find but talking to your gp can help, they are great at signposting to other agencies, mine or me in touch with social services (a bit rubbish) and a local social prescriber or link worker who found the charity/group I've been working with.
Good luck, I hope you'll be able to make a start and will find that you have a lovely home waiting for you underneath all the clutter xxxx