What my hunch is that you have anxiety issue, in anxiety person underestimate his capabilities, think he can't tolerate situation,find it difficult to face situations,avoidance behaviour and physical symptoms like shivering, Burns in stomach . Here it is not clear what type of negative thoughts, are they repetitive in nature or what is theme of your thoughts.
On daily basis we have thousands of thoughts ( positive and negative thoughts) .so it not possible to control all thoughts . This mean we have partial control over thoughts.
we humans have tendency to eradicate the thoughts,if we fail to eradicate ,in that case we try to avoid thoughts by some distraction or avoiding the situations, we also try to lower their frequency . More we try to eradicate, avoid , distract from these thoughts more they come .
there are few ways to deal with them .one is that Instead of avoiding these thoughts accept these thoughts as they are without try to avoid them , certainly presence of these thoughts would lead to discomfort but bear that discomfort. thoughts come and go like clouds in sky , clouds in sky are not permanent, in the same way thoughts are not permanent.
Detach your self from the meaning of these thoughts.
Other way is try to reframe these thoughts. Think how others would look at these thoughts? how would you see your thoughts after 10 years?
Next Steps
go for counselling