This thread has been quiet for a while, but I wanted to share a bit of my experience with my SO. In our case, it was not really about conversion to MMM, but rather to investing, and there's some overlap there, I think.
I can't stress enough what others have said about patience/reasonable expectations, etc. My wife was pretty good at the first 2 steps (making the money and not spending all the money), but step 3 (investing the saved money) was a no-go for her. It was driving me nuts to see years and years of her savings (six figures) sitting in a Chase account making .004 interest or whatever it was, but investing was a terrifying concept to her, pretty much synonymous in her mind with taking the cash to Vegas. That was about 4 years ago.
A very important transitional step to investing in stocks was convincing her to transfer $ to a high yield savings account. Even that was scary to her, because it was new, and the idea of a online-only bank was sketchy/dubious in her mind. It took time, but I did convince her to transfer almost all of her savings into a CIBC Agility account, making 1% interest, or a bit more than that. I did this by 1) setting up my own account first and showing her the process 2) explaining the concept of FIDC insurance 3) having her transfer a small amount first (under 1K), letting it sit there for a week or 2, and then transferring it back into her Chase account - this proved to her satisfaction that the new account was not just stealing her money, and that the money would still be there, and accessible.
My wife was ready at that point to transfer about half of her savings to the high-yield account. A few months after she did that, I had her look at the latest statement. For her, being able to see with her own eyes what her money was doing - making $50-60/mo in interest, and seeing the balance grow... that was a lightbulb moment. She compared that to the 50 cents/mo or whatever she was making in interest at Chase... and wanted to transfer every remaining penny immediately! Huge step.
It was another patient slog to get her to invest in the market, but the investing foundation had been laid. I slowly introduced the idea of low-fee index funds, trying to show how yes, perhaps they were a gamble, but a much, much better gamble (long term, at least) than leaving all her money in a savings account. After all, keeping all your money in cash is essentially gambling that inflation won't ever happen, right? How's that worked out historically? There was a lot of learning on her part - at that point, she had no idea what a fund was... investing in the stock market only meant investing in individual stocks, as far as she knew.
The idea of dollar cost averaging was also very reassuring for her - no, she didn't have to plop her life savings into a total market or S&P 500 fund all at once... She opened a Vanguard brokerage account (which was easy because that's where her 401k was already), and set up automatic investing - $1K every week. Patience, yes. But the automatic function was so, so important - she didn't have to think about it, or actively make a decision every week - it just happened in the background. That account now has $70K in it.
She didn't hesitate a few months ago when I brought up the idea of contributing to her IRA - she promptly moved $6K. Lately, she's been asking if she shouldn't invest more, now that the market's down! (My advice, and I do say advice, because I've always stressed to her that ultimately, it was her money to do with what she wanted - she made that money on her own before we got married, etc., was to stay the course and keep DCA'ing... But I was really impressed that she had that thought!)
I'm really proud of my wife's progress, because it hasn't been easy for her - growing up, her family culture regarding money was... we don't talk about it. And she's gotten so much better at being able to talk about it. She's also seen the difference in me - as she's gotten more competent with finances and has invested more, I've become more relaxed about money, which she certainly appreciates. It's been effective positive reinforcement; the more she learns and takes an active interest in our family's finances, the less stressed I am, and I am honestly more pleasant to be around. A very nice cycle!
I've picked my battles. We'll never be "true" mustachians - we don't budget, and splurge unnecessarily on some things (restaurants and Amazon, oh my). But I've decided that as long as we're consistently saving at least a third of our income AND being smart (investing!) with the money we've saved, I can live with some sub-optimal financial decisions. My wife doesn't really understand the allure of RE, but she totally gets the FI piece, and that's the much more important element to me!