Assalamu alaikum Ustadh. If a person did a very major sin (Ex:-missing Salah) and if he cannot possibly forgive himself for this type of a sin yet he keeps falling into it from time to time. Is there any hope that he'll be forgiven, As it's lack of regret? And any advice how he can leave this sin?

Without a doubt Allah is the most forgiving. never ever have these thoughts. As for regret don't make this your concern as the shaitan manipulates u into thinking u have none. Why did u ask? Because u don't have regret and don't care? No. But u have to from today never ever miss your salah. You really can't spare 5 mins of your day? ... don't let the shaitan manipulate u.