I am of the opinion to expect my dogs to get arthritis in old age and Hattie at 12 is getting on just a wee bit, so don't feel guilty, she's been getting suppliments and now painkillers.
Now this piece of advice (maybe unwanted) isn't easy to carry out, it takes lots of practice. I highly recommend the second choice.
Number one - you worry about every fall, trip, sleeping for longer spells. Feeling guilty because she can't walk as far, so she can't go with you on some occasions, by choosing this you spend the years worrying and never make happy memories, just memories of the misery of the time spent worrying.
Or choose choice two and teach yourself to accept Hattie is older and be mindful of every precious moment you have with her. Learn not to think of what may happen but what actually IS happening. Tomorrow she may sleep a wee bit longer but today she's running about and you can't tell the future. You know Hattie better than anyone and you will know when she's had enough exercise, walking ect.
Please try to live with Hattie as she is, a happy dog that lives in the moment and create happy memories of the precious years left.