I am new to K2 so please do not judge harshly.
I imported several articles from the joomla! content.
I also made several new ones in K2. The problem is that when I try to link something in a new item to an existing one using the article tab from the editor, I only see the items i imported and not the new ones I made in K2.

Any suggestions??

You may be seeing joomla article references that are left in your system.
There was a jce plugin called k2 Link Item or similar that was written originally by Nicholas of Akeeba and i think updated by JoomlaWorks. Search the Extend K2 section for it

Thank you for your answer.
I tried the plugin and still no result. I can only see the articles i created in joomla! content and not the ones from K2