I think what has happened is the following:
- Most English speakers would say the clause as If I'd lost you (however they might write it), and thus should pronounce it as
- But the stressed /-dlɔst-/ syllable in /ɪfaydlɔstyu/ is very hard for an English speaker to pronounce.
- The normal result of (1) and (2) is insertion of an epenthetic shwa between /d/ and /l/, separating the cluster and producing /ɪfaydə'lɔstyu/.
I am a native Midwestern US speaker, and although I never say If I would have instead of If I had, in fact I rarely say either, and normally contract to If I'd. And /ɪfaydə'lɔstyu/ sounds perfectly fine as a contraction to me. It does not unpack for me into anything except If I had lost you.
However, grammar school grammar being what it is in the Anglophone world, other people's parsers unpack /ɪfaydə'lɔstyu/ in various ways. Some believe, for instance, that it should (or might) be unpacked as If I would have lost you -- and some even believe that this use of the modal auxiliary would is in fact The Conditional Mood, label and all.
And I know of at least one other case of someone's unpacking it as If I had have lost you, which makes my teeth itch.
Generally what actually gets said is contracted, or otherwise subject to some kind of conversational shortening or fast speech rules. Then it gets written down and improved by scriveners, and those naughty contractions gotta go, leaving imaginative reconstructions behind.
Mutations like this are just one more way that English syntax grows.
Resistance is futile. You will be assimilated.