You sound so frustrated, and I can't blame you. You went through an easy adolescence, and then your skin flares up when you'd think your problems would be over.
It seems pretty clear to me that your problem is internal - there's only so much you can do with topical treatments to lessen the severity of the breakouts, but it's not going to have any impact on the formation of those pimples. So you're treating a problem after it occurs, rather than preventing it. So there's definitely an imbalance going on.
In my experience with supplements, it's not effective to use a single ingredient or vitamin or mineral. It's very very difficult to identify just one item that your body is missing. I realize that, to look at vitamin aisles and "health food" stores, you'd think that's the best route to go -- those vitamin companies make all their money by convincing people to stock up on a whole bunch of individual nutrients and to try to figure out what they need. You stock up, you try one of these and then you add one of those, and it takes years to work it out (if you do at all), and meantime the companies are declaring record profits! Wellness is a huge industry and while it's good for us to be proactive about our health, we aren't food scientists and we don't necessarily do a good job at it.
So, of course, the "Evening Primrose" people are going to tell you that this supplement is essential. That's their job. And it's possible that there are benefits to it, as there are to many plant-based remedies. However, even natural things can have side effects and can make an existing imbalance even worse. People even get themselves in trouble by trying to dose themselves.
In my experience, people do much better with re-establishing balance if they use a comprehensive supplement and good advice & follow-up. There are very few companies that do that but I know of a few, and those people do much better. My own skin has been fantastic and I'm routinely mistaken for being a lot younger than I am, so I speak from experience!
Be very careful using any supplement if you don't know where it's made or under what conditions. You want something made in the US (and not outsourced and just distributed here). You can also look at something manufactured in or approved for sale in a country with even stricter standards than the US (Australia for example - if their TGA approves something and inspects the manufacturing plant, it's likely to be very well made). You can also look for a US government patent (providing it's safe and effective and unique) and the heavy use of clinical data, as well as testing before, during and after manufacture. I know that seems like a lot of work, but in my line of work, I've seen so many people get themselves in trouble medically by self-treating and diagnosing with a miracle treatment.