Try setting aside a specific time each week to handle your post. Open one letter at a time. Make it a routine, may be with a cup of hot chocolate?
I used to use budgeting apps. Not quite up to date but I’m sure there’s some good ones. Also, automate bills, credit card payments and savings - direct debits/ standing order.
Re ruminating, it might help to journal about your feelings regarding your past and future. Write down what you’re grateful for in the present and what you hope to achieve. Talking to a friend as often as you need?
Car maintenance -delegate/ pay someone to do it for you. A reminder on phone for basic things that you have to do yourself.
At work, start each day by identifying the top three tasks you want to accomplish. Break larger projects into smaller steps to make them more manageable.
Use planners or digital calendars to keep track of appointments and to-dos. Develop a daily routine with time for organisation - may be, when drinking your morning coffee?
To love yourself better start by acknowledging your strengths and achievements, no matter how small. Practice positive affirmations and try to challenge negative thoughts when they arise. Consider engaging in activities that you enjoy and that make you feel good about yourself. Do more life - gardening, hiking, being with people, paint blobs on a canvas…
Right now, I’d start by sitting with a notebook and a pencil. Do you have a nice, unused one?