Felt Invisable again today at work

For me it means that everything, people, projects at work, conversations, accure with out any recognition to me being there.
I don't like to cough at work because my boss yells from the next room over, "Are you sick?"
Do you have a cold?" "You better not give it to me, I can't afford to be sick."
Which translates to me as, "We can get along very well without you," "But I am important."
So when I am noticed it's an accusation.
How childish this sounds, but today I felt that I was untrusted. She told me how a co-worker had told her I was not using proper protocols in the database. Pretty wide open statement and when I tried to answer her I got, "I do have time for this."
I have issues with self-esteem and am in the habit of making (in a circle) things to come around to my fault. Is it because I don't stand up for myself? Am I hiding from asserting myself in work issues? I work part time and have been there a year. Is there some trigger I can used to stop feeling invisible? Do people feel that part time employees are less important then full time?
I need some advice. Thank you

There could easily be a hierarchy going on, and it's probably safe to say that your lack of assertiveness is playing a role in how people behave toward you.

Interpersonal Effectiveness

6 Signs You're Not Assertive Enough and What to Do About It (scroll down)
This site offers help on other subjects such as self esteem (left toolbar).

great site thank you