You could also simply have a low threshold for stress.
That’s my problem. If everything is going well, I feel great - I’ve got the situation down, I’m walking on sunshine, life is good, everything’s coming up roses, here are a few of my favorite things and the whole nine yards. 
However, when I get into a stressful situation, or somewhere that I’m not comfortable, I suddenly flip out into this godawful pessimistic Debbie Downer/Eeyore morph. I hate my life, I’m fat, I’m unintelligent, no one loves, or even likes me, and if they do it’s because I’m a total fake and they’ll find out eventually and discard me like the sad sack that I am… and on and on and on.
Anxiety is a bitch.
And it doesn’t even have to be a BIG anxiety - some minor screw-up or incident at work will have me worrying and flusterating for days!
That whole “be positive!” doesn’t really help, because it’s focusing on externals - be positive and you’ll get x! be positive and x will shower down on you! be positive and watch life improve. It didn’t really help me because I was stuck at the “be positive” half. I didn’t even give a shit what it would do for me - I wanted to know how to DO “being positive” when I spiraled down into my funks.
For me, here’s what I eventually learned that actually helps:
Meditation first, to learn to shut my damn mind OFF. (my flavor is ki-Aikido, but lots of people say that Yoga is really good - I’m the reverse of flexible, so Yoga stresses me out, which isn’t exactly the goal here.)
Learning to recognize when I’m being irrational about stress and worry. (Husband and I call it “mudballing” - the image is like that comic image of a snowball rolling down a hill and getting bigger and bigger, but it’s mud instead of snow.) Just recognizing that I’m doing it lets me take a moment and consciously redirect and think about something else, or find a funny video or comic strip to read, to lift my mood a little in the moment.
Physical exercise. It’s really hard to be depressed/anxious when you’re outside in the sunshine, and even HARDER to be so when you’ve tired yourself so far out that you can’t even think straight, let alone obsess over anything.
Plus, the nice physical rewards help counteract the stupid thinking “I’m fat, I’m ugly, I’m unmotivated, I’m uncoordinated, I can’t do anything” because looking at my much more fit self, I can SEE that it’s not true.
Giving myself permission to vent and rant and rave and get things off my chest, and then not bringing them up again for a certain period of time. Both halves are useful, but they work best together. I used to have a set time, but I’ve gotten better at this one, so I pick time-delays now based on how upset the situation/stressor makes me. Varies from a week to a few hours.
To me, this does two things. The venting bit acknowledges that whatever it is actually IS a legitimate worry, and that I do have to deal with it, and I don’t want to. Then, I have to find other things to talk about/do/handle, because I am not allowed to talk about that problem any more for a while. Putting it away like that minimizes how important and terrifying it is, because here I am making dinner and talking about the cats, so the world CAN’T be ending due to x. Sometimes now I even find that when I get to the end of my waiting period, I don’t feel the need to mess with x again - it’s like it’s resolved itself while it was hanging out in my subconscious.
Don’t know if any of these will help, or seem useful, but a bad stress reaction could be one reason you’re such a different person now from when times were good.