I started suffering from depression when I was 19. I have no idea if I had been suffering from depression long before that and I was either unaware/in denial of it or if it didn't officially manifest until then. It began affecting my life pretty gradually as pessimism, low self-esteem, low sense of self-worth, & all around just flat out sadness/apathy became a regular part of my day-to-day existence.

Depression doesn't just disappear. Right now you might feel okay, but you're only 21 years old, a time when mental health issues get triggered very easily. Do you really want to get back to your lowest? If not, it's time to take precautions now, open up to someone and find strategies to recognise possible triggers and work out strategies to manage your depression a bit better.