Hi there. I'm not a teacher but an native english speaker. With the ed, it is used in past tense. You HAVE considerED - that is you already have done the action of considering what ever it is. You are considerING something if you are about to do the action or are doing the action. Example: I am considerING to eat my dinner (I am thinking that I will or will not eat my dinner). I have considerED to eat my eat my dinner (I have now decided that I will eat my dinner).
Hope this helps.