This is one of the key skills I learnt from my course of CBT. It's ok to be upset, anxious, sad whatever for a short time but if you dwell on it that's when things tend to get worse. Dwelling on the negative sends you into a spiral and that spiral is a lot harder to get out of. For me there were daily spirals that would last 10, 15 minutes maybe even half an hour or more. And every day that happened, the feelings inside me were building up and the cloud above my head gradually got bigger and bigger and harder to live with to the point where not being alive seemed like a solution. There's science to all this - your brain chemistry is constantly developing and re-wiring itself and if you are exposed to certain feelings over longer periods of time it will affect the types of thoughts and feelings you have because you are more/less prone to positive/negative thoughts and feelings.
I've not posted much in here recently but have been checking in on the thread. I hit my absolute lowest point in May this year but it turned out to be exactly what I needed. I've had a short course of therapy, I got a new job after a month off sick and I moved back to a city where I am surrounded by friends. The biggest thing of all though was the influence all of this has had on my mindset/attitude and how I've been able to help myself thanks to some help from professionals and people I care about.
Life is pretty good for me right now. I hope that anyone reading can take some solace in this and try and move forward.