When you use simple past tense as in "I lost my keys this morning" it means that you lost your keys and then you FOUND them (the action began and ended in the past and is somewhat irrelevant to the present).
In the second case, "I've lost my keys" means that you HAVEN'T FOUND YOUR KEYS YET, because present perfect describes an action that commenced in the past and IS STILL HAPPENING.
So I lost my keys and I found them, and I've lost my keys and I haven't found them yet.
Make sense? What other tenses are you struggling with?