What should I do about the problem of excessive flatulence that prevents me from maintaining ablution at night, knowing that I like to be in a state of purity when I sleep?

Maintaining purity (tahara) before sleep is recommended unless it causes hardship; otherwise, there is no harm in sleeping without tahara.

As for gases, if they stop for a time sufficient for tahara and prayer, then this is not considered incontinence (salas). However, if they are continuous and do not stop for a sufficient time, then the person experiencing them is considered among those afflicted with incontinence.