As a long term investor, how do you mentally deal with volatility and FOMO?

I’m 25M and started investing heavily for the long term last year. Maxed my ROTH IRA last year and this year and I have other brokerage accounts after maxing my ROTH. I want to do a big lump sum investment of $1500 but I have the fear of FOMO because I believe that the market will go down but I also have the fear of maybe the market will go up and I’ll lose my opportunity. I know that I shouldn’t and can’t time the market but mentally I can’t stop thinking about it.