Why can't I stop these thoughts?

I think we all go through a period of time where we think we aren't worth anything. Sometimes these feelings are a result of low self esteem or because of shame. The most important part of dealing with this thought is to separate feelings from reality. "I don't feel worth anything, but I know what I do is important, I know who I am matters to others. I know I am loved and cared about."

When you find yourself having these thoughts, stop, take a few deep breaths and try to reframe what you're thinking.

We all suffer from random thoughts from time to time. Being able to manage your thoughts and choose what to think about yourself is something you would learn in  counseling. An effective treatment modality for this situation is cognitive behavioral therapy. 

A lot of times the thoughts we have about ourselves are messages we have received from other people and later internalized them. So in that sense, these thoughts are yours and belong to you, but you have received these messages from somebody else. Even though we all receive messages like this at times, it pays to be careful not to ignore other messages we are receiving, such as a loved one appreciating your help or attention, a friend responding to encouragement, or a colleague showing interest in you. I hope this helps. 

We have thousands of thoughts a day. Some thoughts we choose to think, and some thoughts are random as you describe. We can't control the first random thought but as soon as we become aware of it we can make a conscious choice to change the thought. Positive affirmations are messages we intentionally read to or tell ourselves to help us counteract negative thoughts. If you find yourself under stress on some days more than others you may notice an increase of these negative thoughts about yourself.  To help you neutralize your negative thoughts and make it more natural to randomly think positive thoughts about yourself you might find it helpful to work with a therapist who specializes in cognitive behavioral therapy. This professional can help you build insight into your errors in thinking, help you develop a stress management plan, and provide you with additional tools to boost your self-esteem. 

Many people have thoughts like those you describe, and often it feels like someone else is saying it because they are things that may have been said to you when you were very young.  When young children hear negative things about themselves they tend to internalize these negative ideas and to form negative core beliefs.  The good news is we can learn to stop these thoughts and to replace them with healthier thoughts.  The first step is to catch yourself when you are thinking these thoughts, and to stay "stop!"; then replace it with another thought.  So for instance, maybe you fail a test or get rejected by a romantic interest.  You catch yourself saying "you aren't worth anything". Stop this thought, and replace it with "You didn't do as well on that test as you would have liked.  Let's figure out how you can do better next time."  Or "she may not want to go out with you, but someone else will". So the idea is to develop a voice of a "friend", who can tell you the kind of things you would say to your own best friend.  You might also pick up the book, Feeling Good, by David Burns. He gives many tips for how to change Negative Self Talk.

Life is beautiful without unwanted thoughts and stress. With proper strategies and tools it is possible to regain control over your thoughts. I guess you are just having "thoughts" and not actually "hearing voices"? My suggestion is to see a therapist and go from there.

First off, there is nothing wrong with you.  In fact, having random thoughts we feel we cannot control is actually quite common and normal.  

Sometimes our thoughts think they are protecting us (if we think we aren't worth anything, it won't hurt when we get rejected) but they are actually doing quite the opposite.  They are keeping us "stuck" and creating a self-prophization.  (If I think I am not worth anything, I don't have to try, and I will keep proving to myself that I am not worth anything).  

There are many techniques to work through and start to change our negative self-talk.  There are actually many self-help workbooks that can help with this as well.  

A technique I like to use is meditation or mindfulness.  This can teach us to accept our thoughts (not fight them) and then learn to let them go.  Release the power they have on us.  If we can learn to release these and not ruminate on them, this will give us the space to allow more positive and supportive thoughts to come in.  

Mantras can help with this as well (I AM worth a lot, I AM important, I have value).  Saying these to yourself everyday over and over (even if at first you don't believe) can retrain our brains to believe it (and allow us to believe it).  I have even wrote positive mantras to myself in my bathroom mirror so I am forced to look at these throughout the day.

Please remember to not be hard on yourself as you begin to attempt to change the automatic thought patterns.  It took a while for you to get where you are and it will take a while for this change to kick in and feel normal.  So allow the process to happen slowly and allow yourself to accept you as the amazing and brave person you are.  

Those  critical thoughts most likely came into your mind as the way you understood what one of your parents or some other adult who was very meaningful to you, at a very young age.

When we are too young to distinguish whether what a grown person tells us, feels accurate, we absorb their opinions as our own.

Your question shows you've reached the point of emotional maturity to know that your opinion about yourself is not the same as what is inside your emotional brain and stored there since you were too young to know that someone else's opinions are not necessarily your opinions.

Nothing is wrong with you, everything is right with you to wonder how to more firmly establish your own point of view.

Start the habit to revise any viewpoints about yourself which don't feel true.

Then replace these thoughts with better ones which you decide on your own.

After a while of doing this you'll either automatically think the more positive thoughts about yourself or at least be in the habit to know that negative viewpoints about you are not necessarily true.