Ish called Iner.. Inar... Ener... Enert... Inertia, my friend! In space, nobody can hear you break, because you can't. Logically enough, getting two mechs moving at totally different speeds (relative to the nearest star) close enough to each other for more then one melee attack (or any melee attack at all!) is extremely difficult; as it should be.
The only contraption suitable for melee attacking in space with a mech would be, say, a starship.
That being said, if the lack of gravity has got you down (Or HASN'T got you down, Haha!) one can hit "5" on the numpad to "stop". I'm not sure if that would aid your melee attacks in any way.
If you absolutely, positively MUST melee in space, I suggest using an EXTEND weapon, because that extra space (I love this) makes a huge difference. - Space combat adds a lot of pressure (Kekeke!) to the game, oddly enough.
Lastly you could try haywire missiles or something. Perhaps that would subdue their evasion.
...On the other hand, since this is a mecha rpg with anime influences, perhaps melee in space should be easier despite the illogic? o.o; I wonder if you can get a Space-Rat-Flail.
EDIT: If it is only the turning driving you nuts: before you wish to turn, hit 5 (or 'stop') and then rocket away in a new direction.