Hi! let me helps u...
i would say playing a slow tempo piece is more difficult than playing a fast tempo piece, haha, it sounds weird right? if u don't have technical problems, of course u can play a fast tempo piece easily, but... but... a slow tempo piece, the speed shouldn't be dragging but it still in slow tempo, it's going and going without stopping, normally slow tempo piece will always make ppl feel sleepy, so what can we do for attract ppl's attention? as if walking slowly in a beautiful garden? (not like a car get stucked in traffic jam) first, u must have the correct pulse in heart, u can count it out of mouth if u like(count in pulse, not in quaver or crotchet beats), once u get the pulse i think u will be fine. i have the same problem as u, i tend to be faster and faster when i felt excited... so if u get distracted in counting while playing, i would suggest u to play with metronome, but don't rely on it, just for few times of practicing... cos it will affect ur playing sounds very metronomic... like robot! so once u get the speed, then play without metronome... haha... make the notes 'sing' would be better...