Hi, I'd like to help you with your question.
It is a matter of addressing your fear as a singular feeling rather than as individual fears. When people have anxiety and fears, they focus their fear on any number of objects, people or situations. For example, some who is fearful may fear cats or they may fear bridges. What they fear may be different, but the feeling that causes the fear and the answer on how to resolve it are the same.
What you need to address is why you feel afraid and how to cope with it. Anxiety and fears are, at the root, about how your body reacts when you are faced with what you fear. Adrenaline is released when you feel the fear (you feel you are in danger) and this causes you to react as if you were. That reaction is what causes most people to develop fears. The become overwhelmed and feel they are going to die or have a heart attack, both of which are far from the truth. The way to counteract this is to learn how to control your reaction, where it comes from and how to see it for what it is- harmless.
You mentioned that you are in therapy and on medication. Are these treatments helping you? The therapy alone should be helping you to feel better. Talk therapy has shown to be highly effective with anxiety, phobias and fears. If you feel you are not better, you can try another therapist. Finding a therapist you feel helps you and that you are comfortable with is much like finding your family doctor. It may take a few tries, but you will click with someone.
Also, because fears and anxiety are so common, there is a lot of self help available for you. Here are some resources to help you get started:
The Anxiety & Phobia Workbook by Edmund J. Bourne
Anxiety, Phobias, & Panic: A Step-by-Step Program for Regaining Control of Your Life by Reneau Z. Peurifoy
The 10 Best-Ever Anxiety Management Techniques: Understanding How Your Brain Makes You Anxious and What You Can Do to Change It by Margaret Wehrenberg
You can find these books on Amazon.com or your local library may have them for you.
Let me know if I can help in any other way,
If you have pinpointed a root cause for your fear, such as an abusive parent telling you you do not have a right to exist, then you need to deal with your fear from that angle. In other words, you go back and work on the emotional abuse you suffered from hearing that being told to you. That usually entails therapy work to resolve because it is deep seated. It can be resolved, it is just a bit more complicated because it involves relationship issues and unfulfilled needs and desires.
If you somehow developed this fear as a child with no discernible reason, it is a matter of confronting this fear with the truth. You need to practice thought stopping and replacing this thought with the fact that you do have the right to exist. Also, it helps to go over the facts of the situation. For example, who says you do not have the right to exist? If someone did say it, who says they are right? What proof is there? And why do you lose your right to exist if others have the right to be here? What makes you different? These questions need answered so you do not have any way to refute the fact that you do have the right to exist.
Each time you have the thought, say stop in your mind and replace it with the thought that you are worthy just like everyone else and that you do have the right to be here.
What you are dealing with sounds like possible Obsessive Compulsive Disorder. If a therapist or treatment professional has not evaluated you for this disorder, consider getting another opinion. Although it is hard to tell just from talking to you on line, the possibility needs ruled out. If you would be diagnosed, it would make your treatment much more effective.
I haven't heard from you. Did you have more questions or want clarification?
OCD can manifest itself through thoughts that are distressing or external behaviors. Either one or both are acceptable as part of an OCD diagnosis.
There are no books on the right to exist specifically. Most of the materials available for OCD though would be helpful to you. Here are some recommendations to get you started:
Overcoming Obsessive Thoughts: How to Gain Control of Your OCD by Christine Purdon and David A. Clark
Coping with OCD: Practical Strategies for Living Well with Obsessive-Compulsive Disorder by Bruce Hyman and Troy Dufrene
Freedom from Obsessive Compulsive Disorder: A Personalized Recovery Program for Living with Uncertainty by Jonathan Grayson
You can find these books on Amazon.com. or your local library may have them for you.
You can improve your belief of your right to exist by addressing the basic issue here, which is the OCD. Getting an evaluation to confirm the diagnosis would be a good step in helping yourself. Educating yourself on your diagnosis and working towards healing is the next step. Everything you do to work towards getting better will chip away at the thoughts you have been having. Proving that you have a right to exist is great, but it would most likely not settle your doubt because, like you said, you would always be able to come up with a reason why you should not. So addressing the main issue would help resolve the thought since that is where it originates.