Well, I believe there is a difference between taking responsibility and ego.
I'm definitely a terrible example myself, but the two should be completely seperated in your mind.
For example, when your sensei asks you to help and lead in class, don't think of it as "Is it becuase I'm good now?" Take it as "I am below my sensei so I will obey his order to help lead." This keeps both the humility and responsibility intact. After all, who are you to question your sensei's orders? This switch of perspective could help; I hope it does.
Oh, and about this:
But I feel the need for humility and for the lowliness i'm used to as kohai, so i don't allow myself to appear as competent as I could be.
Don't think that your competence reflects you, think of it as representing you sensei! Be and look at competent as you can and represent your dojo well, then if anyone wants to praise you, give all credit to your sensei and your seniors! Other than that, always see yourself as below your sensei instead of above your juniors, and I'm sure you can find comforting humility in that.