Flavors in Fresca

tl;dr: Grapefruit oil is the most likely missing link. Squeeze some grapefruit peels into your mix, or pick up a "food-grade" or "therapy-grade" essential oil (for extremely sparing use).

In the absence of more information, I'll go ahead and take a broad pass at this. As I mention in comments, more data will help us get closer. As a starting point, let's have a look at the ingredients in Fresca according to Wikipedia:

1. Carbonated water
2. Citric acid
3. Concentrated Grapefruit juice
4. Potassium citrate
5. Potassium benzoate and EDTA (preservatives)
6. Aspartame
7. Acesulfame potassium
8. Acacia
9. Natural flavors
10. Glycerol ester of wood rosin
11. Brominated vegetable oil
12. Carob bean gum

[note: numbers added for easier reference]

Some of these are pretty easy. #1-3 are pretty obvious and already present in what you've tried. #5 are simply preservatives, not really essential for a homemade version. #6 and #7 are no-calorie sweeteners, frequently used in tandem. You can obviously use sugar if you're okay with that. #10 and #11 are both agents used to keep oils in suspension in water (more on this later).

That's the relatively-easy stuff out of the way. #12 is a bit odd, but it's simply a thickening/gumming agent, probably there for mouthfeel and to replicate the slightly "rounded" flavor that sugar would provide. Now, we get into the trickier items.

Potassium citrate (#4) is simply a weakly acidic salt. Again, from Wikipedia:

It is also used in many soft drinks as a buffering agent.

In other words, its likely job is to keep the overall pH constant even in the presence of #2. However, it also has a slightly saline taste. It's unlikely that this contributes any perceptible saltiness at all, but as I noted in another answer, salt can have a suppressive effect on sourness and bitterness as well even if it's barely detectable itself.

Acacia (#8) is tricky because it's not clear whether the seeds/extract are used (presumably for flavor) or if it's actually gum arabic, which is more used for thickening (similar to #12). Gum arabic does appear in many other soft drinks for texture, but I find it odd that it's not identified as such here. It could be a form of "inferior gum" from other acacia species, or perhaps it is actually acacia seed. These are described as having a "chocolate, coffee, hazelnut flavour profile" but again I find it odd that cheaper extracts wouldn't be used in a commercial product like this, if the primary intent is flavoring. My money is on acacia gum for thickening.

Which brings us to the hardest of all, that catch-all term: #9, "natural flavors". This is basically their way of keeping the exact formulation secret. However, we might be able to draw an inference from the Mexican version, which has a slightly different formulation:

• Carbonated water
• Sugar
• Concentrated pink Grapefruit juice
• Essential grapefruit oil
• Potassium citrate
• Potassium benzoate and EDTA (preservatives)

Grapefruit oil (most likely extracted from the peel of the fruit) is the major difference here, and the only thing that could approximate those mysterious "natural flavors". It's also the reason for the presence of #10 and #11 - to keep it in solution. If I were to suggest one possibility for the missing link, this would be it. A quick Google shows a wide variety of available versions, many of which are labelled "therapy-grade", i.e. for aromatherapy. These can supposedly be adapted for food use by diluting in a "carrier" oil (they're very potent, with some important cautions for their use) and are sometimes used in candymaking or baking. I often see many similar oils and extracts in the baking section of high-end cooking suppliers.

You could try purchasing a bottle of extract, but you might never wind up using it all, so I'm going to suggest a much easier workaround. These oils are extracted from grapefruit peel, so why not use that? Buy some nice ruby-red grapefruit and use a vegetable peeler to strip them of their outer peel, being careful to include as little pith as possible. Prepare your juice/sugar/water mix in a wide-mouthed container, then gently squeeze the peels just over the top and drop them in. Let this steep for a couple of hour, stir, and serve. You could also keep some extra peel handy and squeeze those over the top of the prepared glass - the aroma adds a lot and they make a nice garnish too. If the mixture seems bitter, try adding a very small amount of salt solution, which will suppress the bitterness and bring sweetness to the forefront.

Again, this is a pretty wide-ranging answer. I'll do what I can to help with a specific recipe if you can add in exactly what you've tried that's not working.