Phrases like 'You can do it' just bounce off if your mind has ten counter-examples ready. Self-confidence isn't built by encouragement, but by experiences you remember.
Keep a list of evidence
One line after every training session: What worked today? Concrete, small, verifiable. "I was ahead of my opponent on three out of four retreats." After four weeks, you'll have something stronger than any gut feeling.
Drawing correct comparisons
Comparing yourself to others tells you something about the competition, nothing about your development. Comparing yourself to you six months ago tells you something about your hard work.