How to overcome the guilt of not serving Krsna properly?

Nothing is more purifying than the holyname so you have to purify your external and internal senses, so just remember, meditate on holyname and Krishna then you will be perfectly purified, internally and externally.

om apavitrah pavitro vā sarvāvasthāṁ
gato api vā yaḥ smaret puṇḍarīkākṣaṁ
sa bahya abhyantaraṁ śuciḥ
śrī viṣṇu śrī viṣṇu śrī viṣṇu

When you chant the holyname then bahya and antar, external-internal shudhi happens.
bahya abhyantaraṁ śuciḥ, purity, one becomes pure. So remember and chant Krishna’s holyname. On top you can ask for His forgiveness. Anything that we cannot do, “Krishna, please forgive me. I’m duty bound.” So you can say

aparadha-sahasrani kriyante ‘har-nisham maya
daso ‘ham iti mam matva kshamasva madhusudana

You pray to Krishna, “Please, forgive me for my short comings” and Krishna is very compassionate. Our God is very compassionate, His whole existence is compassionate. Krishna is the embodiment of compassion. When you pray to Him for your shortcomings, immediately, He helps you, forgives you, He comes to you and doesn’t mind anything, very compassionate.