An honest question about staying invested all along?

So I've been investing for a couple of months now, and how do you have the motivation to keep investing even when prices are up? Like I mean, for example, when I started investing into stocks and index funds, my avgs were down and now they've all gone up and so has my portfolio. But everything is expensive. And are we assuming it will always go up eventually over the new few years/decades? like to infinity? Or do u guys rebalance portfolio every once in a while(1-3 years) switching to different