You need to do things gradually and not focus on getting it perfect but 'tidier' at first - if you have a lot on the floor for example, make it a goal for that one day to just get the floors looking more spacious. What about selling some stuff too? That might give you a good incentive to sort stuff out - according to eBay people have up to a grands worth of stuff in their house which they could resell. I've sold a lot of stuff recently and it feels great, I've cut some painful emotional ties and it really feels like I've moved on. Just keep focused on the tasks at hand rather than worrying about the end result, it always feel
overwhelming when you think about the bigger picture. All the best.