In addition to the great advise previously, it could be that your scope is hard to focus because it hasn't been collumated properly. This is more noticeable when you use shorter focal length eyepieces. An easy to understand analogy of this would be making a small, already grainy picture larger. It will be even more obvious that it is grainy when it has been magnified.
Collumation isn't really tricky once you know what you are doing. Plenty of good tips around this forum. It should be done often for best results, some would say every time you use your scope.
*just noticed you have an astromaster 130EQ in your signature block. That has a focal length of 650mm. So, your 6mm eyepiece on its own gives you a magnification of 108.333X. The barlow doubles that. Now because your aperture is 150mm, you should be able to push your scope to 300X using the rule Andrew talked about (but remember the
perfect conditions caveat. Light pollution, the moon, turbulence all affect the seeing). Given that I have confirmed it is a Newtonian Reflector you have, definitely have a look at the collumation and compare the view you have to what it looks like around the new moon.