When I first started to suggest it to my wife I went through the same yo-yo roller coaster ride as you! She would get high or tipsy and it would be 'yes, I'll do it', but then when she cooled off it was 'no go'. That went on for a long time and I had settled for pretending by using a life-like dildo. I nicknamed it by her ex-boyfriend's name...a guy whom she admitted had satisfied her thoroughly and was well hung like the dildo. I thought it would never really actually happen and then one day she came home telling me about a guy at work that had been coming on to her. I got her to admit she was attracted to the guy. I reminded her that the offer was still open and I had no problem letting her go out with the guy. She never said she would or not, either way, but then one night I got a phone call telling me she would be late coming home...she said she would give me all the details at a later time...asked if I was ok with it. I asked if it had to do with what we had talked about and she said yes, but she didn't want to go into it too much on the phone. My mind was racing 800 miles an hour!!
I had finally hit a home run!! She went out to dinner with the guy, and the rest is history.
The best tools in your quest are PATIENCE and PERSEVERANCE. Push her slightly, but not too much...back off when you have to, and keep reminding her that the offer is ALWAYS open, and you would love her all the more if she ever decided to go through with it. She may eventually take that leap of faith as mine finally did. She later confessed to me that she had feared I only wanted it so I could have an excuse to be with other women myself...once she became sure that it wasn't the case, then it became easier for her to accept the lifestyle. Now she (and I) both love it, and she's free to fuck whoever she wants...just so long as I know about it and I either get the details from her, or a ringside seat to the event. Good luck...hope it works out for you as well.