Can we have sinned so much/too much, that we can no longer return to God?

Just because we have the light of Christ, does not mean that we are able to completely understand everybody and in every situation.  We are not Gods.  We can only try to act like and be like Christ (since our God sent Jesus Christ to this Earth to show us the pattern of how to be like Him or like God).

Don't get ahead of yourself and remember that no matter what, you haven't gotten so far away from God that you can't return or can't reach Him...  Better yet, do you really think that you can get so far away that He can't reach you?  Are you more powerful than He is?  Are you more powerful than God?  Are you so powerful that you can damn yourself so much that even God would be unable to save you? Are you yourself omnipotent?

No, you are mot omnipotent.  But God is, and He is always there for you (remember that He is also omnipresent and omniscient, to name a few more...) and He has sent His Son to help us to get back or return to Him (because we are all sons or daughters of God--children of a Heavenly Father).  Jesus Christ is mercy embodied.  He is so merciful and charitable that He holds the arms of Justice back to allow anyone of His Father's children to return to God's presence.  Those arms of Justice will hold back everyone that is the least bit unworthy of the Celestial Kingdom (any sin, any uncleanliness, any whatsoever...), yet because of Jesus Christ, and only because of Jesus Christ, those who follow Him and obey His commandments, and repent can and will enter into His presence!

In answer to the original question, though, just don't deny the Holy Ghost and you'll be fine.  You may have to pay for some sins, but your faith and reliance on Jesus Christ will do you right!

JPS