I'm a long-time Lib Dem voter and sometime activist (no longer, only because I emigrated).
I first started to worry during Cleggmania in 2010, when it turned out after the first debate that he'd got a lot of his facts wrong. He seemed like another Blair. Significant, because Blair was all about posturing, and to my mind, that's been a significant failing of the Labour Party since long before Blair's time.
Bear in mind that Cleggmania didn't actually do the party any good, even during that campaign. Its share of the vote in 2010 was 23%, precisely 1% up from 2005 and it actually lost a net total of 5 seats. The only significance of Cleggmania is that, for once, the traditional media briefly took proper notice of the Lib Dems. Gee whiz, a daily newspaper actually supported the party.
Then there was that frightful bromantic Rose Garden vignette where Clegg and Cameron launched their great supposed love affair. I think the Lib Dems were right to go into coalition with the Tories. Given the state of the country's economy at the time, the alternative (ie, Tory minority or second general election) would have been irresponsible. As others have pointed out, Labour didn't have enough seats to form an overall majority and thus any alternative coalition or other agreement would have needed to involve at least another 2 parties, including the DUP. An agreement with Labour would have been by far the preferred option amongst most party members, but it simply wouldn't have worked.
Where they went wrong was pretending that they agreed with the Tories on everything. I'm guessing it a misplaced reliance on cabinet collective responsibility. is Oh, and the press quickly took a few scalps. I still don't understand why David Laws got off so lightly, I'm dismayed at the broken pledge on tuition fees, and dismayed at how naive they were.
The thing that dismays me most of all though is that the LDs in government ended up presiding over a country that relies on food banks and being a party that constantly rattled the sabre of austerity as if it were some kind of masochistic virtue. It's just awful. Just the opposite of what we were working for. Just about all LDs I knew saw themselves as liberal rather than left or right, but they did see that society had a responsiblity to support the individual, rather than leave them to rot.
It's encouraging to know that the membership numbers are going back up; hopefully the constituency parties can rebuild and at least get back some of the losses in local government.