This is typically the case with antiques. They go through popular periods when the prices are high, and lulls where you can’t sell them… Take brown furniture right now, beautiful Georgian pieces are going for a song at auction houses in the UK, whereas 20 years ago there was strong demand.
I find that the best thing to do is to focus on items that you really love, decide what you want to pay for them, and don’t go past that. If you are a dealer trying to make a profit it must be stressful, but as a collector who just wants to enjoy the pieces I buy then I am more relaxed about whether they hold their resale value.