I also made a decision not to have a relationship again when I was 53 a long time ago , after many happy times in long term relationships...but being let down/hurt in the end. I cannot do it again, far too painful and I value my mental health so no one is going to threaten that.
I live alone, want to be left alone most of the time too, and enjoy my own company, don't go out much, never meet new people, am older...and mostly content with that. I'm an introvert though.
It's been liberating, taking pressure off, lessening stress and the freedom is wonderful. I have lots of interests but often think 'Is this it?' too. If it is, I accept that, I've loads of good memories and many are widowed/divorced etc by my age anyway, it's life.
Therefore I constantly motivate myself to do something different, learn something, go somewhere new, find new activities. But deep down I've always been like this and it sounds like you have changed OP, which you might be finding upsetting as you sound as if you have been more extrovert.
I've never made a partner the passport to being happy, it has not been the be all and end all to me....too independent.
It does take a long time to adjust to loss and life changes and to grieve so maybe you will find the old 'you' again as you eventually find life can be good. You can do whatever you like to make you feel fulfilled and at peace, it's all out there. Make plans for the spring/summer and find a new activity, set small goals. I love the simple things in life primarily, and enjoy them each day.
Your life is not over, it's waiting for you.