Gf seems to have lost interest

Hi
I was in exactly the same position.
My wife of 16years didn’t take it well when I told her I wanted to share her!.
I gave her time for it all to sink in, and a few things helped..
I got her comfortable in playing a game whilst we were out and about, she became comfortable rating guys out of 10, then the game evolved onto which ones she’d fuck.
It worked because it was only fantasy, so she felt no pressure, although to me it was very real!.
It made her believe it would hurt us, and that I actually wasn’t jealous, these were her biggest fears.
I moved onto letting her know she could if she wanted to, which did give her a buzz, not that she said that, but I could just tell, maybe it’s the excitement of her just knowing she could.
The next thing I tried did involve little white lies!…
I told her jokingly that a good male friend fancied her.. He’d not said anything, but I knew he did.
She did have a smile when I told her, so I spoke to him one day and in confidence told him that I was positive my wife fancied him!… Which she obviously hadn’t said.
I then put the cat amongst the pigeons by inviting him around more often.
Months past with them clearly flirting, with my wife even going commando under her skimpy t shirt and PJ bottoms.
I’d joke with my m8 to enjoy checking my wife out, and said the same to my wife.
The sexual tension built over time, so I moved onto telling him I’d like him to fuck my wife, and telling her I’d like her to have fun and have sex with him.
It all got to the point where he’d turn up with them both knowing they were allowed to fuck, and both wanted to.
We got as far as arranging a night at his place where they were going to have sex, but at the last minute he bottled it, saying he’d hurt his shoulder.
To this day he says it’s his biggest regret, but it showed me it was possible to get my wife laid.
I told her to just find a different guy, and over the next year or so she slept with 5 men, many multiple times.
I’d suggest not giving up, but try and keep it fun, make games that your both involved in.
Not only will she start trusting your idea, but when you plant that seed in her head that she can and that you want her to, well, she will carry that thought wherever she goes, she will look at men knowing she can, you just have to wait for that perfect guy to come along.
Periodically ask her if she’s met anyone she fancies, she won’t tell you, but may hope you ask first, then remind her she can!… The temptation will grow.

Very insightful strategy, Shaun.