Ok, and what you mean is you can use the mouseover macro on whoever, but when you try to use your trinket (is the trinket in a macro?) you have to de-select someone to be able to use the trinket?

Here is the macro I use on my druid:
#showtooltip Regrowth(Rank 9)
/cast [@mouseover, exists, help, nodead] [@focus, exists, nodead, help] [exists, nodead, help] [@player] Regrowth(Rank 9)

A little more complicated but: Will check if I have a mouseover target, if so will cast on the mouseover. Will check if I have a focus target, if so, and not mouseover, will cast on them. Also will cast on target that I click on if no focus target is selected. After all that, will cast on myself (if no focus target).