What you buy
does matter, but it's hard, perhaps impossible, to know what that is going forward from any given point in time.
Periods of "irrational exuberance" sometimes last an irrationally long time. 1990's were a good example, as has been the last 3 years under Trump. Missing out on them really diminishes your gains. Hanging in too long guarantees big losses. My own experience in 2008 was I bailed completely about 1/3 into the rout & went entirely to cash, but then took too long to get back in ... which meant I ended up at exactly the same point after about 4 years as if I'd simply ridden it all out.
Diversifying helps minimizes the possibility that what you pick is the wrong thing. It may limit your potential gains in the short term, but will also limit your potential losses. It reduces the gambling aspect & combined with a long term horizon makes it almost certain that you will realize solid gains over time. It's like gambling - but being the "house" rather than the individual gambler. Having said that, the deck is stacked against the individual investor, so that is a negative factor.
Diversification with a variety of indexed funds is the best way to minimize portfolio depleting fees, but it requires a lot of work to keep on top of everything. I tried index funds for a while - it works out well when the market is going up, less well when it's going down.
I've lost a lot of money (on paper) over the least month.
It seems like we are at an inflection point right now. The understood economic ramifications, (plus some extra fear & panic) have been factored into the market already. That would appear to be flat growth in the 1st quarter, a big contraction in the second & a smaller contraction in the 3rd quarter, followed by strong growth in the 4th quarter. My concern is: people in the US seem to be blithely unaware of, or ignoring, what is happening in Italy & in other European countries. This suggests that in 3 - 4 weeks time there will be a health care crisis in the US. This may mean that thing might get a lot worse & last a lot longer than the market is anticipating at this time. Against that, there's the uncertain prospect of an immediate "miracle cure", which if it turned out to be true would turn everything on its head in short order.