A core tenet of successful wealth generation is to have a long-term outlook and stick to your guns with investment portfolios. A good quality, well-considered investment portfolio will do well if given the time to do so with (hopefully) minor tweaks along the way to keep things current.
Unfortunately, human behaviour can play havoc with this.

Many investors try to pick the bottom of the market to buy in, or high points to sell, and this involves making investment decisions based on short-term market movements. In contrast, a buy-and-hold approach involves buying investments and holding them over the long term, irrespective of movements in investment markets.