It's going to happen because the stock market periodically drops.
Any investor will tell you the worst moments are watching your money literally go down the drain. Do I sell? Do I REALLY sell? Oh wait, they always say not to panic.
That's correct, there is a very strong temptation to sell off something, at least I'll salvage some money because at these dark moments (say 2007) it can look like there is a major collapse. And, sad to say
during those two years it was collapsing because underneath stocks and bonds were some terrible mortgage loans that did go to hell.
Try taking stock of your situation. Do you believe yor or your advisor has done a decent job of diversifying your holdings? Is there a reasonable balance between stocks and bonds or bond funds? In most online accounts you can look up your mix of investments. In truth if you opted for a fast moving heavy-on stocks approach you will be taking a ride because that's what dives fastest. But if you have some bonds or bond funds very likely those moderating (not fast changing) bonds will help keep your equilibrium.
If you have an advisor talk with them, listen to their ideas. If that advisor suggests selling everything and buying replacements you must ask why? The original investments should have been sufficiently sound to handle steep drops if you are patient.
Finally, does your advisor frequently trade your holdings? Almost certainly you are witht he wrong person and they are making added fees from these trades. Time to reassess your decision to work
with this person. And, just guessing, this person is really a stock broker not an advisor. You can consider buying a handful of index funds, Vanguard developed the original index funds and they are still totally valid investments. Start with VTI, Vanguard's total stock market index fund. Yes it will go up and down because it is actually the whole market. But generally large scale index funds change more slowly and tend to be stable.