It's impossible to time the market. If you are a value investor and see a stock that's undervalued, it's perfectly ok to invest as much as you want right away. In all other case, investing regularly and over time averages the cost. Holding on to it for a long enough period of time gives a consistent and better return.

What kind of sum? Were you doing dollar-cost averaging?

This kind of shit stressed me out so much to "track" I gave up entirely. Not like I'm ever pulling it out anyway.

edit: https://www.investopedia.com/terms/d/dollarcostaveraging.asp

I felt the same way so I just started investing a litte each month instead

That way it all kind of averages out instead of trying to time the market

You only lost a month's worth of "progress", the dip doesn't seem like a big deal unless you wanted to sell something today.

I sold everything in 2008 at the lowest price, had I not done that they all come back eventually. I think it just took a few years to come back as it has been all the time, which is why the saying goes, "don't use the money you will need for the next 5 years to invest", otherwise it should be fine.