A flower in full bloom

[SimikPK]

Hi.

A flower in full bloom can't stay,
Yet knows not that it must go; The idea is good, but at the end of both lines you have highlited the same thing - the necessity of going, while there are potentially other things that could be highlighted - the fact that it is the flower in full bloom, or its ignorance of the fact of having to go...
As perhaps one might decline each day The "perhaps" maning-wise, is redundant here 
And of life's cruel design not know.  These two lines make sense only if you elaborate on them - on the "one" and "the cruel design"

But I, not blessed with falsities,
Fall victim to despair. 
Oh, how sweet the ironies
That I, success won't spare. As ellajam said, what ironies, what success, what despair, what falsities? A stanza filled with abstract nouns which sounds very vague, victim to despair is semi-cliché to me. Being more specific about what is the situation of the "I"of the poem would make it more tangible. Try showing some specific success, some specific irony or falsity, that the reader might grasp more of the state the "I" is in. 

The cause of my mortality
Reveals itself, when applied, this "when applied" is very confusing, it seems that it is forced there to rhyme with subside. 
As my own mentality;, 
And to this, I must subside. Noting new said here - you already told the reader that the "I" is "victim to despair" and that "success won´t spare". This stanza says basically only: I must die because of my mentality and the only new thing is the mentality. Be more specific about the mentality, and make the reader understand it, for, so far, I dont. What is the mentality like that it causes death? 

[edited to fix typo, originally mentality said mortality]

 

Also, why do you begin each line with a capital letter? There is no need. It is not less poetic if you use fullstops, commas, semicolons etc. and not always begin with capital letters. 

The poem has much potential to grow - like the flower. Just be specific, not too abstract, and show us more, we do want to look and see.