TBC 1382 That's what I get for looking aka The UPS Man Saga Part 1

Here we go, folks, yet another sorted tale of woe.
Speaking of yet another, here is yet another apology for the length of this post.
I (40'sM) found out that my GF (40'sF) of nearly 10 years has been cheating on me.
To some degree.
With the UPS guy?
GF and I have been together for over 9 years and living together for over 6. She has a son who I love as if he were my very own son.
For the first year and a half, our intimacy was gangbusters, if you'll pardon the expression. That was until 1/4 into our first year when I ran into some financial and emotional difficulties. We powered through it and were stronger for it.
I really thought I had found someone that would stick by me through thick and thin. Someone I could really open up to and be myself, which is something I had trouble finding up until that point.
Yet, it was then, now mid-way into our second year, that our intimacy began the inevitable decline from the honeymoon phase. By year 3, sex had gone from several times per week to several times per month. As she was an amazing GF in every other possible way, I didn't let it get to me that much. Even when we were "like rabbits" she was never what I would call a sexual person. For one, she refused to do oral on me in any way, even though I was happy to do so to her.