I self-identify as liberal and not progressive, for the most part, but I still consider myself to be on the left, not on the middle.
For the most part, my politics are similar to progressives - higher taxes, public healthcare, more expansive welfare (I support UBI), and social justice in general.
The reason why I don't consider myself a progressive is that I feel that the progressive ideology in US has largely abandoned the traditional concept of natural individual rights in pursuit of all these goals. You have people openly claiming that freedom of speech is not worth it if it gets in the way of criminalizing offensive speech, for example. Or how due process is an outdated notion that makes it hard to prosecute sexual harassment properly. Basically, the further left you go with progressives, the more individual rights are sacrificed to collective ones.
That, to me, is the line past which you can no longer meaningfully call yourself a liberal, since liberalism is all about individual rights, ultimately. Where left-wing and right-wing liberals differ is on the matter on how to best maximize those rights, and to what extent government regulation is advantageous or detrimental to that, especially on economic matters.
And, to some extent, on what those rights are, precisely - for example, I don't consider property rights to be fundamental natural rights (but then again, neither did Thomas Jefferson, and he sure wasn't left-wing!). Note, it doesn't mean that property is a bad idea - I think it's a social construct that is broadly useful, it's just not an absolute right (and hence taxation and wealth redistribution is not inherently an infringement on such).