It feels to me like a clash of personalities. That may not be a fair statement. But......at the least........ a conflict of choices.
In all that you and your late husband wanted for her perhaps she has been somewhat detached from everything you and your husband did. Sometimes it takes longer for us to understand who we are growing up.
I was certainly detached from my mother's interest in becoming a JW. Though I did ........live that life for a time.
When I was 18 I met my future wife.....I though she was 17 but she was 15 1/2!!
We waited a year and a half to marry. She was the oldest unmarried child in her family and they were starting to get worried........ LOL.
We have been together for 54 years now. I told her this morning that it was my privilege to wake up every morning next to her..........don't know why I even said that. But we have that kind of relationship...... a hug passing by one another in the hall way. Respect for one another.
We have a wide circle of friends and when they learn how long we have been married they ask......... what did you do when you guys married so young.....she tells them the truth....."We continued to raise one another!"
The odds are probably against you resolving this situation or that things will work out. But they just might.
Step back and trust your daughter to find her way....... and if she fails....... be there for her. Make sure she knows that........ because you will be her everything.
No one way to live, no censure or shunning........ you don't get off that easy........ from what you have written you are not that weak. You have a fierce love for your daughter.....but now she is an young adult and it is important to make adjustments in your relationship going forward.
The JW religion is not the issue....... it's just a temporary foolish distraction. She can leave it at any point and return to her family........ which is you. Make sure she knows you are there for her.