i suffer threw depression, but right now, idk why i am depressed?

So ive suffered through sevear depression for about a year now.. But i normally always had a reason, bad break up, family loss, friends leaving ect...

But right now i have a great gf bothing is wrong between us as far as i know, i have loads of great friends and my family is happy and thriving like it used too...

And yet im still deprrssed, not as much but still a lot.. I even feel suicidal, like the world would be such a better place without me in it... And idk why i feel this way...

Why do i feel this way? And what can i do about it?