I'm not sure if it was a mistake or just a failure of will, but Sanders started off with a strategic blunder.
I suspect Sanders thought, "I'll run for the nomination as a Democrat. I know I won't win, but I can get enough votes to make the Democratic party progressive. Therefore, I have to promise to support Clinton when I lose."
If Sanders had won 20% or 30% of the popular vote, his strategy would not have been a blunder, but sill would have been a mistake.
This strategy could not work. The Democrats cannot be "moved" away from neoliberalism, not from the inside. If Sanders had won 20-30% of the vote, he might have had a seat at the table, but he would have had very little actual influence. He might have bumped a token minimum wage increase up by $0.25, or shaved a half-point off of student loan interest, or provided a small increase in PPACA subsidies, but he would not have made the Democratic party progressive, just a little less neoliberal. Maybe that's all Sanders wanted, in which case I would fault him for a lack of vision and will. But I don't think so: Sanders has been outside the Democratic party too long for him to just want to be a mid-level Democratic operative.
If Sanders had really wanted to actually implement a progressive agenda, he would have had to take a risk. The only way to enact a progressive agenda would be to run as a Democrat, but at the same time build a third party to nominate him when Clinton wins the Democratic nomination. Then, during the general election, Sanders pushes hard for a progressive agenda; either Clinton moves hard to the left, or Sanders splits the Democratic vote and Trump wins.
There are four possible scenarios.
First, Sanders gets 5% of the vote in the Democratic party. He drops the third party and goes home. Progressivism is just too weak. Maybe next election, with someone younger.
Second, Sanders gets 20-30% of the Democratic vote. I suspect he thought that if he ran as a "good" Democrat, he might get 30%, but if he ran as a "spoiler", he would get only 5%. The problem is that, as noted above, getting 30% as a "good" Democrat is not much better than 5% as a spoiler. But if he gets 20-30% of the vote as a spoiler, he's in a position to mount a serious third-party bid and could actually force Clinton to the left. However, he could also pull a Nader and split the liberal vote and scary Trump! wins.
Third, Sanders actually wins the Democratic nomination. It almost happened. Then he runs as a progressive Democrat and wins, because scary Trump!
Fourth, Sanders gets what he actually got: more than 43% of the vote; he probably lost only because the fix was in. As a "good" Democrat, Sanders represents an existential threat to the party. Not only does he not get a seat at the Democratic table, after the election the party will quietly purge Sanders and his supporters, either literally or by forcing his supporters to pay obeisance to neoliberalism and abandon all that progressive nonsense. So I call his decision a blunder: not only will he not force the Democratic party to the left, he has actually given them the incentive to move to the right: force the progressive out of the party; they'll be too disorganized and weak to mount any kind of a challenge.
As a spoiler, however, he's really got a shot at not only forcing Clinton to the left, but actually winning the general election, because scary Trump! and nobody actually likes Clinton.
I suspect (again, I can't read his mind) Sanders thought he could get at most 20% of the vote, win a few delegates, and have real power inside the Democratic party; furthermore, he thought that if he got 30% of the vote and ran a third-party campaign, he would have thrown the election to Bush or Cruz or Rubio. (I'm sure he was as surprised as anyone else that Trump actually won the nomination.)
Progressives do not, I think, realize that politics is always and everywhere a chicken game. To win a chicken game, you must convince your opponent that you'll crash before you swerve. Sanders was not willing to risk a crash (Republican victory), so he literally promised on day one to swerve. Thus, his only chance at a progressive victory was to actually win the nomination, which would have been nearly impossible even if he had run a perfect campaign and progressive voters were a majority of Democrats.
By promising to be a good Democrat and not leading a third-party challenge to split the vote, he gave away progressivism on the first day of his campaign.