Early in our attempts, we often use days, years, or time periods to get us started. For a while, my goal was just making it past 3 days, because 3 days was as long as I could go without a drink. Oddly, I didn't enjoy any of those three days. In fact, each one of those days was pure Hell, and each day got worse. Eventually, I made it past 3 days, but nothing got better, and 5 days was the worst day I experienced in recovery. And then I experienced a shift in my thinking when an old timer pointed out that the real goal was never taking another drink, not reaching for a specified length of time.
Like you, this forever idea was something that seemed beyond reason. If three days were murder, how much worse is forever? But I tentatively decided to stop marking time and focus on what I needed to do to avoid alcohol. Like you, I had doubts, but I decided for the first time in my life to at least give "forever sobriety" a try, because it was obvious that what I was doing was getting me nowhere. And the good part was that I had nothing to lose. Things got better fast, and soon I was in the "I'm done with alcohol for good" mode. Many of us simply call this freedom. THAT is the goal!
There is a paradox we need to understand here. When one year sounds like an absurd possibility, it's actually easier when you go for the bigger absurdity of NEVER. That's when things started to fall into place for me. I say started, but there's lots more that needs to be done, although its the fun stuff, not like those early days of nagging cravings.
Of course I support your 90 day experiment, but if it doesn't work or starts to seem like it won't work, think about changing your sobriety goal to "all in" as they say at the poker table. But unlike poker, when it comes to sobriety, "all in" is a pat hand, and a risk free bet. You can't lose going all in when it comes to sobriety. Sure you could fail, but so what? All you've been doing up to this point is failing anyway, so no matter how you spin it, you won't be worse off than you are now by going all in.
But try it and see. Try it now. If you do succeed, what seems like silly prattle from a stranger will actually make sense one day.