ohhh complicated. what on earth are you studying.
lets start with metaphysical, it means to consider the existence of someone or something. The metaphysical poets for instance (John Dunne) were a group of poets who were completely wrapped up in considering their own existence and in the reality of the world around them. their poetry held to one idea or line of argument with in the poem.John Dunne for instance seemed to be constantly thinking about his own death and his own existence and the reality or lack of reality of himself..... should appeal to you!
A conceit however is an extended metaphor, that is where you compare one thing to another stating that a thing is something else. So if I was to write a metaphor of the sun I would say the sun is a big ball ( simplistic but it will do) if I then wrote an extended piece of poetry on the same theme that would be a conceit. but also the comparison would be strangley alike and unlike at the same time and the skill is in making it seem both and if I wrote a poem with the theme of existence of something or some idea running through it in a metaphor using my skill to show that the two ideas were cleverly puttogether in a juxtoposition then that would be a metaphysical conceit.
So if we look at John Dunnes poem death be not proud
DEATH be not proud, though some have called thee
Mighty and dreadful, for, thou art not so,
For, those, whom thou think'st, thou dost overthrow,
Die not, poore death, nor yet canst thou kill me.
the metaphysical conceit is in the fact that death is being given an existence, being turned into a character. and the whole poem revolves around this supposed battle with death
hence the metaphysical conceit is the extended metaphor about the existence of death as a character or person.