If you're looking for manga similar to Ressentiment, you might like these titles. All recommendations are made by Anime-Planet users like you!
Ever since the Pharmaceutics Law passed, everyone in the country has unlimited access to a dizzying array of mind and mood-altering drugs. A shot of Slumberine helps you sleep, and any emotion you can imagine is just a pump away. Even vacations can be replaced with shots: take one dose, and your mind is bathed in a relaxing hot springs experience. Amidst all this drug-addled merriment is Kabu, a world-weary twenty-something man, with nothing to do but chase his next big high. But one day, after running out of P, his drug of choice, he experiments with a new super-drug, Ultra Heaven, which completely re-writes his sense of reality and changes his life forever...
Though not particularly similar in terms of plot (Ressentiment is a depressing story about otaku, while Ultra Heaven is a slightly-less depressing story about drug addicts), both are seinen soft science-fiction titles that explore a near-future world where one particular facet of life has been highly developed (video games/drugs).
Neither have terribly likeable protagonists, nor are light fluffy reads. Additionally, they both have a similar split between events that are actually happening, and stuff that's going on either in a video game or in the protag's mind. (The third volume of Ultra Heaven in particular is a lot like the rpg-world of Ressentiment)
For years, paranoid Satou Tatsuhiro has shut himself away in his apartment for days on end, with barely any social contact whatsoever. Then one day when he answers the door, he is greeted by an older woman and a girl called Misaki, who are going from door-to-door to inform people of the recent social problems of hikikomoris. In a fit of depression about his way of living, Satou decides that the only way to escape his current life cycle is to muster up his courage, face his fears, and go outside to find a job. However, instead of employment, he finds Misaki waiting for him; she intends to make Satou her ‘project' so that he can reconnect with society. Now with the help of his friend Yamazaki and Misaki's evening meetings, will Satou be able to escape his hikikomori lifestyle, or will he simply fall deeper into the clutches of conspiracy and his own demons?
Another seinen manga about otaku/hikikomori with a similarly cynical approach. Both are pretty bleak, but while Welcome to the NHK! covers a huge array of awful things that can happen, Ressentiment is more focused- people who liked the eroge and MMORPG story arcs in the beginning of NHK! the best are the most likely to also enjoy Ressentiment.
Life begins at 40… even for pathetic losers. Shizuo Oguro is living his dreams… sort of. A complete waste of a human life until now, 40-year-old Shizuo breaks free from the corporate rat race and charts himself a fairly random and new career course: to become a published manga artist. Sure, he lacks the talent, discipline, or any other skill necessary to become a success in the manga industry—but that’s not enough to stop Shizuo!
Want to read another seinen manga about a middle-aged(ish) loser who's doing nothing with his life? Ressentiment is a sci-fi story about otaku and virtual-reality dating sims, and I'll Give It My All is about a single father who quits his job in a (very poorly thought out) attempt to become a mangaka, so while the plots aren't terribly similar, both have a focus on nerd culture and protagonists that will make you feel really good about yourself in comparison.