Welcome to HW!
First I'd like to point out that depression doesn't need a cause. Sometimes you just feel bad. And judging by what you wrote, yours does have a "cause", when your husband is away. Or maybe that isn't the cause. And I don't know if you blame yourself for being depressed for such a reason, but don't. Blame will only make it worse, it doesn't help.
I would advise you to seek some counseling as well, as those are trained professionals who will be better at helping you out of it. Please remember that some forms of depression can be treated with medication, so that might be something to look out for as well.
To state it again, depression doesn't need a cause, and accepting your depression (as you already have done) is a great first step to take for making it go away.
I wish you all the best, and if you have any questions, or just want to chat, I will be here.
Acceptance is the key
Existential depression and Insecurity
Try to keep smiling! :)