Feeling hopeless, Hard lump in breast!

Hi everyone. I am feeling a hard lump in my breast. I'm almost 21 years old and am feeling very frightened. I have no family history, but I started birth control at 16 years old because of bad periods and I was sexually active. I first noticed my lump in July of 2017. I was taking a shower and there it was! It felt as hard as a bone. It appeared to be large and is located along the outer edge of my left breast, right near my sternum. It is in the middle/upper part of it. It is tender when I press on it The hardness hasn't let up with my cycles. It has been there for a little over a year! I've been to 4 different doctors to find out what this is, and they've all given me clinical breast exams, only to say it's fibrosis tissue or a fibrodronema. (I think that is how it is spelled?) I've asked for further testing since my symptoms are not going away, only to have them all say, "You are too young." Or "Young women can't get breast cancer." Smh. I've tried being patient but now I am feeling hopeless! I need some input. Should I ignore my symptoms or push harder for further testing? I am not an anxious person usually, but I am very annoyed that I don't have an answer. Can fibrodronemas feel hard and immobile? Is it normal for there to be hardness in that part of the breast?

My other symptoms other than the lump is breast pain, rashes, dimples, flattened nipple,stinging nipple with flaky skin, my ribs ache, and discomfort in my armpit. No swollen lymph nodes could be felt in exams though. Any other young women that have also gone through this?

I'm not going to work myself up too much over this. No need to be overly stressed out over something I do not yet or never will have. Lol I'm trying to figure everything out. In the meantime I will enjoy my company with my boyfriend and our lovely Dalmatian!

Hugs to all! Trying to stay positive.