Don't beat yourself up! You have enough to contend with with what is unfolding and whatever is going to happen, and you will need all your energy for that!
Its easy to wonder "what if...?" but now it is academic, and you have to go on from where you are. Believe me, I've had these thoughts too, so I know!
I didn't have a mamogram for about 4 years. When I did discover a lump it was large, but who knows how long it had been there? I found it cos I had lost weight. I immediately thought it was another cyst, had absolutely no thoughts that it could be malignant, and delayed having it checked because I was going on vacation that month.
When I came back we had the disturbing news that my brother's wife had breast cancer. I told my brother about my lump, and he was "go get it checked out .... NOW!", and I guess it was another about 4 weeks before I got my mammogram, ultrasound, biopsy Oops!!!
Would things have been different if I'd had my annual mammograms or was it a fast growing cancer that can appear between screenings? It really doesn't matter now. It is what it is ...... but I do advise all my friends to go get the bloody mammograms now !!!