It may not help at all , but when my husband was alive there were times when he was away for things on his own and I did likewise. I found when he died, that things that had never concerned me became a problem. So when there were two of us I never thought about checking that the door was locked more than once, or going round checking the windows on the ground floor. But once I was on my own, they became a worry. In the beginning I just accepted it was part of the general situation, but then I firstly trained myself to only allow for testing the door twice and then no matter how on edge I felt would not turn back. So the compromise was that I would do this when going to the local shops so that it didnt take long for the confidence in my own checking to improve. I had a key lock fitted and my son knows the numbers, but so does a local trusted friend, so that I feel if there is any possibility of a medical emergency she can check me out , as I do for her, so there is that reassurance that friends who know your sort of routine would be likely to notice that you had not gone to something quite quickly. Sometimes in the early days I could think that he was away and carry on the way I would have done, but of course you know the reality. However thinking about what you did do when he was away, can give you some help to think what you enjoyed on your own and maybe think of joining a group who have the same interests. If you find something to enjoy, the other thing is that he never went to that group, so you are not constantly reminded that he is no longer with you. Of course it does not always work, and on occasions you have to just accept that it is a bad day, lock the doors, have a cry if that helps , do something comforting - eat an icecream - read some rubbish - have a long bath. Whatever can take the edge off the misery, but promise yourself that the next day you will go out to something, even if it to walk to a cafe you have never visited before, just to have something new to try.
The main thing I think is to learn to accept that this is not a straight road to a different way of living. You can have two or three days when you are able to do various things and try something new. But you can never know which days will be better than others. I found that anniversaries were not as bad as I expected. I suppose that you sort of mentally prepare for that difficult day, but a renewal letter from a magazine he took, arriving unexpectedly could floor me . So I actually do not try to look for any sort of measurement of how things are going. Just make the best of good days and in a very simple way I began to put either a simple star in the diary or an exclamation mark. Star positive , exclamation mark, negative. and I did not try to compare or count them for a very long time. But after a couple of years, when I was very down one time I looked back and was cheered to see that almost imperceptably I was managing to do that bit more. It was no good looking for improvements in the short term, but I could look right back and then even seeing that I popped into a cafe that I had not been in before, just on the off chance was good. The fact that I hadnt had to think and organise it down to the last moment. If you enjoy gardening, I do think it is the most excellent pastime. For me , it has been a lifesaver. However I am feeling , I can find an outlet gardening. Get those secateurs and snip away, thinking of someone like Trump cheers me and lets me let off steam without actually screaming out loud. Weeding gives you a great chance to scrumple up plants and dig about and physically do somethin. At the end of your efforts, your garden will look a bit better and you will be more relaxed, and of course you can look out of the window and see how far you have come. Wishing you all the best