How can I repent and remain steadfast in it, knowing that I saw the Messenger (peace be upon him) in a dream, and is postponing my Umrah trip related to my sins?

Neglecting prayer is one of the gravest major sins. Some scholars even consider those who abandon it, even out of laziness, to be disbelievers. Perhaps the dream you had is an impetus for you to repent and promptly perform every prayer on time, regretting what has passed, and increasing voluntary prayers and seeking forgiveness. The majority of jurists hold the view that missed prayers must be made up. Your claim of inability to consistently maintain repentance is incorrect, for Allah does not burden a soul beyond its capacity. Among the things that help maintain prayer are: going to the mosque, accompanying righteous people, and remembering the virtues of prayer and the gravity of the sin of neglecting it. Postponing your Umrah journey might be due to sins, for Allah may deprive a servant of sustenance due to a sin. However, do not despair of Allah's mercy, for He accepts the repentance of His servants and rejoices in it. Therefore, hasten to repent and abandon sins.