You're overwhelmed and feeling a mental paralysis. Forget the major life overhaul for now and focus on being kind to yourself. Start with a routine.
A getting ready and breakfast routine for the morning. Write it down. Nothing complicated. Eat breakfast, clean teeth, washing up. Wash face and moisturise, brush hair, get dressed. Commit to doing it regardless of how you feel. Then something to cheer yourself up before dinnertime comes.
You mentioned cuppa in the garden. Break the task down, write down each step in a list. So -
find a cup,
find tea ingredients,
boil the kettle,
brew the tea,
gather a jacket/picnic blanket/magazine or whatever you need for cuppa in the garden,
take it all to the place you'll sit,
set an alarm on your phone for dinnertime so you don't lose track of the time.
Then eat.
Then do something in the afternoon. Maybe one hour of house chores or life admin so it doesn't all get on top of you. Again, make a list. Don't worry about perfect, just do something, anything will be an improvement on doing nothing.
Do something relaxing. It'll help with the overwhelmed feelings.
Tea time. Eat again. Try to pick something healthy to nourush your body, but anything is better than nothing.
Have a rest. Read a book, watch TV. If you're feeling tense, exercise can help get rid of the stress hormones.
Get washed and comfortable, put out your clothes for tomorrow, go to bed at a decent time. Whatever your problems are, you can't solve them when overtired and stressed in the middle of the night. So focus on relaxation and sleep.
Repeat day after day. Give yourself time to sit with your thoughts and feelings. Keep a journal of that to get it out. Gradually the overwhelm will lessen and you'll be able to do more and think clearer. I guess a therapist focused on relationship problems would be the one, empty nest is loss of a relationship, a sort of grief maybe, loss of your identity as "mum" now who are you? Your relationship to yourself.