Sometimes worship time is difficult for me. I get into church and sing along, but my mind is somewhere else. Someone hurt my feelings or did something that upset me... something is bothering me... there's something I need to do after church and don't want to forget... there's someone I'm looking for in the congregation. Sometimes I just can't focus for some inexplicable reason, and my mind keeps wandering. You would think that someone who has spent as many hours talking about worship, rehearsing worship, and doing worship as I have would have it all together by now and it would just snap in every time. No such luck. Some days I struggle just to remain interested enough to keep singing. And sometimes I find myself singing on auto-pilot, thinking about something totally different.
I've heard lots of people reference the verse that talks about the fruit of our lips being a "sacrifice of praise". I think maybe we take the word "sacrifice" in that verse into a modern context, when the Bible is an ancient book, and we really have to understand the context the writers were in. A sacrifice to them was not necessarily something difficult or painful (although for a poor person it probably sometimes was). Do you think it was painful for King David or King Solomon to give up a sheep? Ten sheep? A thousand sheep? Of course not. They had tons of stuff; sacrificing one sheep was not a painful thing for them. The sacrifice, to the ancient people, simply was something they brought to God out of obedience. It was worship.
If you're getting the idea that I'm saying it's OK to let your mind wander during worship, you're incorrect. God deserves our full attention and adoration. What I am saying is that if you bring your worship in obedience to God and offer it the best way you know how, to the best of your ability, I am positive that God still accepts it, even if you keep having to drag your sheep out of the bushes and back onto the road and away from something that's distracting him on the way to the temple!
Don't not worship because you think your worship isn't good enough. Keep trying. Keep focusing. The struggle isn't the "sacrifice"... the obedience is.