What Does A Small Lump In Center Upper Part Of Breast Indicate?

Question: Hi, I am 28 year old female. I have a small lump in the center upper part of my breast. It has been there for about a year and a half. In November 2012 I went to a breast specialist. He performed an ultrasound as well as a physical exam and he noted that it was "benign breast with negative ultrasound today". He explained to me that it was dense glandular breast tissue. It is hard to just go on with that since I can actually feel a lump. in March 2013 I went back because I was still worried. Again he stated "PE and US are consistent with normal firbroglandular tissue. At this time I asked if I could get a mammogram to ease my mind. A few weeks later I got one. They had to perform an ultrasound after the mammogram because my right breast is dense. My left breast is not near as dense as my right breast. The radiologist reviewed mammogram and ultrasound and I received a letter a few weeks later stating it was negative. I was happy, but the lump is still there. I went back for a follow up to the breast specialist to go over mammogram results and again he wrote " Her MMg showed a small area of increased density in the central upper right breast. US shows no worrisome findings." I have not been back since. That was in May of last year. I am going next week only because I have worked myself up. The lump does not appear to have changed. I was told by the nurse that it would be there and only come in if it changes. I just want to get it removed so I do not have to feel it anymore. I feel it so much that my breast is sore. Now, I feel like I have a lymph node swollen under my arm. The only reason is because I was digging around looking for something. I have read that breast tissue extends up into the armpit. Now I have made my armpit sore when it wasn't. I guess I am asking if I should worry? is this common? Is it common for one breast to be more dense then the other? With me having 3 ultra sounds and one mammogram do you think it would have shown something? Also, I am not paranoid about my armpit. I have had swollen lymph nodes before due to cuts and shaving and they are big and hurt. I feel nothing like that. I feel like if you feel around enough you will always find a lump. It is so exhausting. I am just looking for profession insight. Thank you.