WieWaldi Metronome helps me to retain speed while practicing, but as soon I play without one: Bam! The problem is back and I speed up.
Have you tried any metronome app that fades out over time? In the beginning it gets you started by giving all or most beats but depending on your settings it can gradually leave more and more of them out and only click occasionally, forcing you to train your internal sense of pulse while still giving some feedback.
TimeGuru does this. There might be other alternatives as well.