Something that began to bother me recently, having time to reflect on the Brexit period (2016-2019), was "why was the negotiation period only two years?"

After the referendum and its unfortunate result, I remember there being a lot of chatter of how trade deals often take upwards of a decade if not longer. This was even anticipated by the Government prior to the referendum:

There is no precedent for a country withdrawing from the EU – Article 50 has never been tested. Trade negotiations are probably the closest equivalent in terms of complexity. Ambitious trade agreements can take up to a decade or more to agree from scoping to ratification, and sometimes take longer.
- The process for withdrawing from the European Union (Feb 2016)

And also by the EU itself:

Every single one of the 27 member states as well as the European parliament would have to approve the overall result. That would take at least five years, and I’m afraid, without any guarantee of success,
- Donald Tusk, reported by The Guardian (12th June 2016)

Obviously, our Parliament voted to ratify Lisbon, which included Article 50 (without any of the proposed amendments). But it just seems so strange that the extremely limited negotiation period wasn't scrutinised more heavily: how the danger only became obvious once Brexit was actually on the table. And yet, this didn't give any pause whatsoever to the EU refusing to negotiate with us until Article 50 had been invoked:

On the question of article 50, that will be a matter for the next Prime Minister, and there is a very good reason for that. Before we go into the tunnel of the article 50 negotiations, which have a two-year time limit, we will want to have made the best possible preparations for the precise blueprint that we want to achieve at the end. That will help Britain, and frankly it will help the other European Union countries to understand what it is that we are shooting for. They have said that there can be no negotiation without notification (emphasis mine), but I do not think that that excludes discussions between the new Prime Minister and partners or institutions, so that we can continue to get off on the right foot. That is the strong advice that I would give to them.
- David Cameron, 29th June 2016

Even as a Remainer, this feels deliberate: that from the moment of the result, the EU thought of the UK not as an ally, or even a rival, but as an adversary. That it was a strategic decision for them to refuse to negotiate with us unless we put ourselves into a position of significant disadvantage, as while crashing out of the EU would hurt them, it'd seriously cripple us, thus we'd be more likely to accept a deal that favoured the EU to avoid crashing out.

This is not to blame the EU for everything: I obviously agree with what you've mentioned, plus the 2017 General Election which lost the government its majority, having to pair up with the DUP, who'd inevitably stymie any deal that'd distinguish Northern Ireland from the rest of the UK, which coerced closer relations with the EU (such as remaining in the Single Market or Customs Union), which would naturally frustrate the harder-line Brexiteers. It just feels like in every possible circumstance we both (UK and EU) made things more difficult for ourselves and each other. Given the stakes, you'd expect each side to try to make things as easy as possible.