Hi Paul,
Recording yourself is one of the keys for improvement. The bonus is that any recording software will allow you to upload a backing track and set any tempo you like without altering the pitch. There are free digital audio workstaions like Reaper and many sophisticated ones for purchase such as Pro Logic for Apple users.
VLC media player has options to slow the tempo of an mp3 as well.
John

If you’re not recording to a DAW (which should be able to slow it down to any tempo) then VLC, as John suggested, is free and simple to use on Windows or Mac.

Hey Paul

I posted some setup instructions for the VLC Player that will allow you to slow any recording down at 1% increments as well as various ways to loop sections

VLC Player looping & playback speed control features

Another app which is a free download is Audacity, it will allow you to slow the track down without altering the pitch.

Richard

I use Transcribe! from SeventhStringSoftware but its not free and there’s a one off payment of about $30 (from memory). It’s designed to help you transcribe your favourite guitar solos from recordings so has all the features you need to slow down and loop sections. It’s also very simple to use.
If you just want to slow down Brian’s recordings then Transcribe! is probably over the top for you but if you had wider ambitions including transcription it might be worth a look.

For MacBooks and iPads, I’ve been using an app called Anytune for 10 years. It does what you want. Sure, I can drag and drop an MP3 file into my DAW, but for purposes of everyday practice, it’s faster, easier, and simpler to just use Anytune. And it’s cheap.