I used the implant for years whilst in a serious relationship. My partner and I also trusted each other and also did not use condoms. I hardly ever had periods when I had the implant in. I worried about pregnancy too but as you said, the doc told me it was likely I wouldn't have periods.
I had it taken out earlier this year because we had planned our first child together, it took two months (and still no period) to get pregnant but I knew for a fact I was pregnant, I felt completely different.
Anyway, I think with so many negative tests and still no period, it's down to your implant and not pregnancy! If you're still worried, maybe this is not the best kind of contraception for you? Maybe you could speak to your doc about trying something else?
No one can predict the outcome of an event with 100% certainty. So, instead of worrying yourself to death... take every step you can to make things work in your favour and just ENJOY LIFE!!!!