Anything which inhibits or interferes with Avodas Hashem is the Yetzer Hara.
If guilt makes you feel sad and potentially lackadaisical for example someone says “Oh i did such a terrible thing I am so lowly how can someone like me present himself before God” then this guilt is the yetzer hara.
If guilt causes you bitterness such that you weep and say “God is so awesome, it is terrible that I have no fulfilled his will, it is imperative that I mend my wicked ways and cleave to him” then this guilt is a good thing.
There are many different opinions on whether or not one should focus on guilt because of the tendency, especially in our generation for guilty people to become depressed.
The Alter Rebbe of Chabad, in his book of advice on divine service Likutei Amarim, otherwise popularly known as the “Tanya” advises that a person should set aside certain times occasionally to make an accounting of the soul and feel bitter about his or her lowly position, but otherwise not to let one’s self be plagued by guilt.
See here for an english adaption of Likutei Amarim Chapters 26 – 28 :
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The above is being provided as a reference. I recommend if possible that the relevant texts be learned inside, with someone who is initiated in these teachings. The reader ought not to make any assumptions or derivations from limited materials and should always consult a qualified rabbi or mentor.