> The idea of venting can be traced as far back as Aristotle, but Freud is the one who really popularized the notion of catharsis. Most of what we assume about the need to “let it out” comes from his assertions about the danger of unexpressed feelings. In the “hydraulic model,” frustration and anger build up inside you and, unless periodically released in small bursts, cause a massive explosion. Starting in the 1960s, this theory was debunked by so many lab experiments that researcher Carol Tavris concluded in 1988, “It is time to put a bullet, once and for all, through the heart of the catharsis hypothesis.”
I think there is some truth to the original theory, but not all "venting" is equal. I definitely have at times experienced growing resentment towards people in my life due to not fully confronting the feelings I had about my interactions with them, but in general I don't think that angry outbursts are a healthy way to deal with this. In an ideal relationship (general term, not necessarily "romantic relationship"), I think honest but empathetic and non-judgmental conversations between the parties is often going to be the only way to truly alleviate those feelings; being able to tell someone why you were hurt by their actions without judging them and then being able to hear the same without reacting defensively is much more effective than complaining to a third party, but it requires a level of trust and understanding that is usually not going to be present for anything but close family members or long time friends. Given that, I think there is value in talking through the issues with a neutral third party, but it can be hard to avoid falling into spiral of anger and resentment, which I think is why seeing a therapist or psychologist is such a common treatment. I don't think that being able to neutrally help someone deal with their feelings of anger or resentment is some kind of superpower or anything, but like any skill, there are some methods that are more effective and some that are less effective, so having some sort of training on the matter generally is helpful. Most importantly though, they still need to build that same level of trust and understanding with the patient, which I think is the most common reason that therapy isn't able to help some people; if you're not open to the idea of learning to trust and get helped in therapy, it's going to be hard to actually resolve anything.