Sorry to hear your news. Yes there is definitely misunderstanding that once your treated, you're ok. I think the chances of reoccurance or metastatic cancer are so much less if cancer is caught and treated early but it definitely isn't a guarantee.
I was diagnosed in 2020 with early bc. I had no lymph involvement at all but my cells were a grade 3. I was stage 1. I had surgery, chemo, radium and hormone suppression therapy but in 2023 I was diagnosed with metastatic breast cancer and now on treatment for life.
Cancer is a sneaking thing and once you have had cancer you need to be vigilant. All you can do now is keep going, get the treatment you need and hope it doesn't return for you. Look after yourself and see a counsellor if you need to. The support here is terrific so good you reached out. Take care