It's like trying to describe the colour blue without saying 'blue' or pointing to anything blue! I have compared it to feeling paralysed or trying to move through mud. Making decisions is extremely difficult and not just big life decisions - I mean deciding to stand up from sitting down, deciding to make a cup of tea, deciding to brush your teeth - the decision switch is always somewhere in the middle. In a sentence, on my worst days I feel nothing about everything. I feel nothing about everything, and if I feel no predictable positive consequence of an action or decision then... why bother?
I didn't understand depression initially. I thought it was just like being in a bad mood all the time but it's drastically different. The way of thinking about it that made most sense to me is breaking it down to a chemical imbalance, whereby there are a large number of factors all changing constantly that have to be managed and will ultimately dictate your headspace. By factors, I mean things like friendships, career/school, relationships, home life, financial situation, exercise/diet, body image, etc.
Sometimes a large change to one of these factors can trigger depression and other times, the chemicals simply become imbalanced because they just do. Just like how a pancreas can inexplicably cease the production of insulin leading to diabetes, neurotransmitters can stop being produced leading to depression.