Hi Liz - I am a step-parent, even though I have divorced my XAH. My step-daughter is now almost 20, and it was ROUGH. We have a better relationship now that she's older.
I believe step-parenting is mostly a thankless job and if you've never been a step-parent, it is impossible to understand. Kind of like, if you've never had an active alcoholic in your life, you can't understand it.
I feel for you, sister.