You can't make these things "go away." The harder you try to make them go away, the harder they pull. That is the nature of OCD. It attaches to your fears and the more you fear it, the stronger it becomes.
SounRAB like you have a lot going on. I would suggest going to a psychologist and/or psychiatrist for behavior therapy and/or medication. I would suggest going to a behavior therapist that specializes in OCD. Specialization is key since they know exactly how the disease works. Its not like any other anxiety disorder.
Just to let you know, I have felt the same way. I have obsessed about the meaning of life, how much time i have, how old I feel (even though I am 20). These are depressing thoughts for sure. You're not alone. I am currently in session with my therapist and am seeing results. They are slow, but I can feel a change. I am also on Effexor which has helped me out of my depressive state. Nobody can function or think when they feel depressed. Might want to look into that.
Hope this helps and stick with it.
~Stuart