I would consider saying something like : "take care of yourself" or "remember that you are loved"...
But these don't work on me, and I'll assume they maybe don't mean much to you either.
Just know that there are people who feel the same way. They share your pain, your struggles. They feel worthless, meaningless. They wish to be gone forever. There are plenty of those people around you.
Tell yourself what you would tell them if you where to meet.
Tell yourself what you think would comfort them.
Try feeling empathy towards yourself.
No matter how many times people tell you that they love you, that they care about you... You're the one that should be telling yourself those words.