'Mere' is an adjective. It means 'insignificant'. 'Only' can be used as an adjective, where it's a variation of 'one'. I think you mean 'merely' (the adverb form of 'mere') instead of 'mere'. 'Only', 'merely' and 'just' are more or less the same as adverbs. 'Merely' sounds more old fashioned than the other two just/only/merely because it's not as common. If you use 'merely' to describe someone else's actions, you might sound condescending, as if you're trying to belittle them.

Only - to indicate single or one of a group or kind: eg I am an only child, used to express a single want trying to minimize the want: eg But Dadddy, I only want a pony, mere - only slight/small or little difference eg, the red car won by a mere .02 seconds.