I am 46 and I also have 100% invested in Brk in my retirement account (401k roll over), and 75% in none retirement.

First, you shall never sell when a stock hit 52 weeks high.

Second, the way I deal with this issue is to allocate some money to buy stocks other than Brk. And ONLY when I can prove to myself that I am making more money from my other stock picks than Brk, I will sell more Brk. So far, it has not easy at all to beat BRK — mainly due to that I am not as smart as I thought I am. For example, this year 90% of my stock picks made money , but one stock (Spirit) lost me a lot of money, making my gain YTD negative. Fortunately, I gained almost all the money back because I didn’t sell any Brk (I even used margin to buy other stocks to avoid selling Brk)