Hi sinkingfast. I used to have similar feelings of feeling ok for ages, then out of the blue, I would start to feel edgy and anxious, and then everything started to feel negative. I knew that this could get out of control, and end up in a depressed state.
In the end I did something called "positive affirmations". It is a structured thing, where I had to write down at least 10 statements, all in the present tense, all possitive. I had to make the statements personal to me, about things I was struggling with.
For example, if I was feeling bad about my confidence levels, my statement was "I am feeling confident and relaxed"
If I was struggling with an activity I had to face but didn't feel like it, like doing the school run, my statement would be "I enjoy walking to school. I feel relaxed and happy".
I wrote at least ten statements (I actually had 20!) and the important bit was to read them out OUT LOUD three times per day. When just out of bed, middle of the day, then in bed.
I also had to write out things before bedtime, things I needed to achieve the next day, such as school run, shopping, phone call, dental appointment, etc. Tick them off once done, and a mental slap on the back.
It worked absolute wonders, I cannot tell you what a difference it made. Years on, I am now naturally a positive thinker.
Try it!