I haven't missed out the last few years, but I did miss out on the 15 or so years before that, because we had a financial planner with a very conservative approach. I educated myself and booted the planner. We keep a fair amount in bonds & cash, so I can sleep at night, and I'm fine with "missing out" on going 100% on stocks because that would negatively affect my life.
I don't care if a market crash happens. I expect it to; I've built contingencies in place for it. And I'm already FIREd, at least for now, so this is very real for me.
There is no "top." There are temporary tops and temporary bottoms, and there will be variations between -- but every single time I've tried small-scale market timing, I've kicked myself for not just going all in when I had the $$ and then forgetting about it.
If the market crashes tomorrow, it won't stay crashed forever. If the market rises at 25% a year for a few years, that won't last forever either. Both are irrelevant unless you need the money soon.
When I get nervous about the future, I go to a site that tracks stock market data for the last several decades. I look at charts: 1 day, then 5 days, 1 month, 1 year, 2 years, 5 years, 10 years, max. Watching all those enormous-looking bumps and troughs even out over time, and seeing that steady upward trend emerge, helps me regain perspective.
I don't know the future. But I know I've got the best strategy I can have, based on past results, which are really the only data we have to go on.