OP, your comment about tying yourself in knots over whether he loves you as much as his late wife...
Let it go 
My widower fiancé adores me 
But honestly - I expect he loved his wife "more" and would trade what we have now in a heartbeat to have her back.
Why?
They had 20 years together. They were happy (she sounds lovely!). She bore him their children. They had the early unencumbered fun years together (no mortgage, no kids). They experienced parenthood together. They bought a first home together. They watched their kids grow. She supported him started his own business, financially and emotionally. They have years of wonderful memories. He also had those final previous devastating intense months together.
He loves me, but not with the same depth.
And that's OK. Our love will be different, and it will grow, and we will experience things together that sadly they could not (retirement!)
I honestly wouldn't be upset in the slightest if he told me today that he loved her "more" - because I'd be shocked if he didn't!
But it's like if he loved her to the moon and back 10x, he loves me that 9x. Maybe it's a little less - but it's still amazing! I don't want to say "enough" because that sounds like settling. He has adored two women - no two loves are identical.
Try to enjoy what you have, and not compare 