I just copy and paste them,
but as for how you are trimming:
make sure that it's set to "grid" in the top left hand corner.
that way it will align everything to the grid automatically.
You will also have to set your grid to samples instead of bars/beats and to what kind of beats you need to adjust to (1/4, 1/8, 1/16, 1/32, etc) if you haven't already.
It's hard to explain where to go to do that, because it's so small,
but it's over near the top right.
That should solve the problem and if not this might also be helpful seeing as you're using loops. (shame on you!) just kidding.
If you hold-click over the tool that allows you to adjust the length of the audio clip (the middle one I believe near the pencil tool)
it will give you more options.
One of them is called TCE (time compression expansion)
This allows you to change the speed/length of a clip without changing its tone.
This is an extremely useful tool,
but you probably won't know what I mean until you try it.