You've tried stropping your straight razor and the edge just won't come back. Your hanging hair test fails and trying a test shave on your forearm is like dragging a credit card across it.
Unfortunately, you need to hone your blade.
Most send blades away for this. You don't send it to a knife sharpening service, because you cannot use a grinder or some other automated process for it. You take it to The Art of Shaving or a dedicated razor company. It'll cost anywhere from $15-25 to have it done.
But if you want to try it yourself, prepare to drop some serious coin on stones and other assorted bits and bobs. There is a small online community of dedicated sharpeners, and I'm not interested in stirring up a hornet's nest of differing opinions, so I'll be pretty general with my advice here.
The general idea is to own or maintain a series of honing stones. You won't be maintaining an axe head or a Bowie knife, so you're not going to want to remove a lot of metal, meaning your stones MIGHT start at a 400 grit for a couple of passes, but quickly move to a 1000 grit, a 2000 grit, or even an 8000 grit.
There are two general types of stones. Water stones are typically soaked in water for a while to get them ready for use, and Arkansas stones are often soaked in oil. Once you use oil, you cannot go back to using water, because the oil permanently prevents the use of water again. The stone must be flat, and if it's not, it needs to be lapped and/or prepared with another stone, or sandpaper mounted to glass. The very best are quarried from China and/or available and quarried from Japan. The Japanese for some reason really value an insanely sharp blade, and have really, really good old world technology around to establish and maintain them.
In contrast to stropping in which you drag the blade away from the blade edge, in honing you drag the blade with the blade facing the direction of travel. The same thing applies in terms of using an X-stroke: once you've wetted and mounted your stone, you place it bottom edge flush with the top part of the stone, and run the blade the length of the stone, dragging across as you go. Rolling the blade over the back of the blade (the blunt end), you make a similar barely-almost-cutting motion back the other way. And once again, you rest the weight of the blade on the stone, you never press down on the blade when doing so.
Do not over-hone a blade. An 8000 grit might take 150 passes or so to establish the blade again, but you typically do this until you feel it "bite" gently into your thumb when you press the thumb (GENTLY!) against the blade. This means the edge is established, and you finish the job from there by stropping.
There are other ways of honing as well: some use diamond paste on balsa wood, for example. But the general idea is to use a very, very fine stone grit to polish the very tip of the blade. The tip is then warmed and smoothed to perfection by stropping. Be extremely careful with this blade at this point.