Getting back with an old ex?

Hello!! I've been single and dating for a while now, but I've been having trouble finding someone worthwhile... I recently started thinking about trying to start things up with an ex-boyfriend, but it's been three years since we last spoke to each other... (The breakup was my fault - I had a lot to learn at the time)

What's the best way to reach out and start a conversation...? Because...I'll never know if I don't try, right?