I also suffer from health anxiety (obviously, I guess, or I wouldn't be on this forum) so I feel your pain. I have a particular fear of breast cancer as I'm BRCA2 positive. It's obviously easier said than done, especially when you have a symptom like a lump in your breast, but give what your doctor told you and your symptoms, I think you can relax. I've read quite a bit about breast cancer and know too much for someone who's never had it, and as far as I know it's extremely rare for a cancerous lump to have pain--usually that's something else, like a cyst. Also, cancerous lumps are far less likely to be affected by your menstrual cycle. and given your age, it's even less likely to be cancer.
Remember that the media loves to scare people (that's how they get people to read and consequently make money) so we hear a LOT more about women finding a lump that turns out to be cancer...it's not too exciting to read about a woman feeling a lump, getting scared, and finding out it was just a cyst. that happens a lot more often but we hear about it a lot less. Given all these things, along with your family history of cystic breasts, as an objective observer it sounds very, very unlikely it's anything but a cyst.