Act like you're already confident
Confidence is often all about mindset and body language. You can make the decision to act confident, and the more you make that decision, the more you’ll start to really feel it.
You could try making more eye contact with others, avoiding apologising for things you haven’t done or aren’t your fault, or sitting and standing with your back straight and shoulders back.
These things can seem scary at first. But after you’ve done it once, you’ll slowly gain the confidence to do it again and eventually, it will come naturally.