Why do i feel depressed all the time?

Did something bad happened before?
Is it because of the past? painful memories?
A possible reason for the depression could be that you've lost control over your life,
and you're trying to regain control over it.
Or someone might have spoken something mean which could have a poisoning influence and could cause the depression.
It could be because of the pain. It might be you are trying to get out of the pain, but there's no way out, and the overwhelming unhappy emotion accumulates, which might cause the depression.
Are you doing something that you don't enjoy doing? Doing something that you don't like for a long time might also cause the depression.
Emotion is a complicated thing, it's difficult to see.
You need to ask yourself where do those suicidal thoughts come from and why you have those thoughts, why they are being created in the first place. Are they part of your past?
Your past can affect your thoughts, what happened in the past is as real as your thoughts.
I think it could that there's something you're not happy with, but it is not fixed but instead it keeps getting worse when people choose to ignore it.
It could be you remember something bad happened in the past(especially when things in the past are related to the present), which could lead to depression.
It could also be an unresolved problem that keep reoccuring that may create the unhappy thoughts and feelings that might be the cause of depression. It could be the underlining problem, see what you can do to resolve it.
Don't try to fight the "darkness". It cannot be fought. It can only go away when you're no longer focus on it and you're focusing on something else. Focus on the negative will only make the negative frequency stronger. Instead of fight it, try to avoid it or try to skip the subject if you can and hold on to the things you love. Skip through those negative thoughts. Leave it behind. Listen to your favourite music. Be happy.
You mention you feel like this constantly. One way to stop it is to quit thinking on the subject.
Once you quit it, you will be free. I find listening to music is a great way to lighten up the day, so you can try too :) And try to do the things that you enjoy.

personal problems?

Depression is not an insignificant matter. Go see a professional, he will help.