In other words, it's scary, and it feels a little too much like a bet to buy-in 100% right now.
But your odds are always best to invest whatever you have to invest now.
The stock market tends to go up. That being the case, you are betting that it won't go up every day you don't invest, which is betting against a very long-running trend.
Lots of people who are paid gobs of money to guess these things sat out all the growth of 2015... 2016... 2017...
You see a risk if you invest today and boom - bear market tomorrow. But what if you invest a little today, and miss gains... a little tomorrow... miss more gains... and so on through the next 6 months, and THEN bear market? You'll have skipped most of the gains to buy extra-expensive stock and you'll still have to weather the downturn.