How to prevent being intimidated by juniors in your group publishing before you
First off: if you are not a full time student, it's natural that you will take longer than others (who I presume are full time students). Don't feel too pressured by this.

Second, if you are involved with your colleagues' papers, should you be a coauthor on them? Especially if this took time you would otherwise have spent on your own projects?

Most importantly, have you discussed your specific problem with your advisor? It appears like getting a publication accepted would be helpful to your peace of mind. Your question has a faint whiff of perfectionism, which may prevent you from submitting and publishing. Your advisor would be the logical person to discuss this with. He should have a good understanding of the expectations in your specific field. If he considers you one of his best students, this is a good starting point!

I would recommend that you write your advisor an email along these lines:


  Recently, some other students in our group got accepted in upper tier
  conferences. I am extremely happy for them, and this motivates me
  immensely to follow their lead. I would like to get something accepted
  in a comparable venue soon. However, I feel unsure of what level of
  quality is sufficient for publication - at what point do I stop
  polishing and submit?
  
  Could we have a meeting in which we discuss some of my current
  projects and work out what I still need to do to get a manuscript into
  submittable form, ideally with a time plan? Thank you!


Finally, I assume you have already read and taken to heart this canonical question, right?