i was exactly the same mate, i'm 27 and had been in mechanics since 16, i wasn't happy for the last few years exactly what you said fitting clutches getting covered in shit freezing cold, 4 months ago i finally quit, went and worked for my mates shop delivering and installing electrical kitchen equipment, dishwashers, ovens, washine machines and stuff, i work 20 hours less a week i'm on slightly less money than what i was but i can live with the difference in pay as i have more time for my pj's, i havn't looked back since i changed job's! everybodys noticed a difference in myself, i'm a lot happier, my hands are clean everyday! no stress, i still do private job's on car's and it's made working on car's more enjoyable,

hope you do well in what you decide to do, for me though it's the best thing i've done