Both Brexit and Trump's election are related to the fact that change processes driven by globalization are accelerating and impacting people's lives. I personally am convinced that this influence has been overwhelmingly positive so far, but there is no denying that free trade and migration are making it more and more difficult to influence anything on a national level. Many people who voted for Brexit imo did so feeling that they were not able to influence politics and outcomes the way it was possible 30 or 40 years ago. And in a way, they are right: The EU does influence law-making, but it is with good reason.
I think Brexit is a mistake and will prove costly and fruitless. I also think many voters were misguided by anti-Brexit media - I had to destroy several threads based on Daily Mail articles with facts, for example. But I understand why the vote came about.
Populism, especially right wing populism, is on the rise in Europe from England to France and Switzerland over Austria to Poland. As a liberal German, I see this as a problem - we know what dark paths this kind of politics can lead to. The European Union has guaranteed peace on the continent for decades. I would hate to see it crumble.
However, there clearly have been mistakes. Expanding to the East and including culturally different Poland, Czech Republic and Hungary was bad, including corrupt Bulgaria and Romania was horrible, and using accession negotiations with Turkey as a tool to keep them close has long been a farce.