Clients and partners are relying on you, and you have to take that seriously. Very few lawyers, especially younger ones, feel all that confident, but some have learned the secret of looking like they do. You'll be ahead of the curve if you act confident and in control in all your professional interactions. Everyone feels it and everyone has a different way of expressing it. Remember, though, there is a line between appearing confident and in control, and appearing arrogant and immature.
Some associates look for reassurance by expressing uncertainty.
This will probably not bring you the reassurance you seek, but likely will just reinforce the feeling in the partner's mind that you lack confidence. Make sure you are as confident as possible about the answers you have been asked to provide before you provide them. Life as a new lawyer can be a real challenge. On the plus side, the work can be immensely interesting, and the opportunity to solve a client's problem and provide a useful service can be thrilling. Achieving smooth interactions with your colleagues within the law firm can sometimes be far more daunting than actually practicing law.