Firstly, you deffinitely have some better shots than you’re giving yourself credit for, shooting skateboarding is notoriously difficult of course.
As others have mentioned you need to expose for your shadows and not the highlights with negative film (something you just have to get used to coming from digital, where the inverse is correct)
Otherwise I’d say just really try to think about other ways to try & create contrast within the image:
Composition - there’s nothing worse than a distracting background, unfortunately some spots will be difficult to shoot for this reason. You might find an angle which really compliments the trick but is just too messy for an image to really work. For instance with this shot you could have shot it like half a second later which would put the subject on a slightly cleaner background, as displayed on my highly accurate diagram below. (however you would also then not have the guy in the background perfectly positioned within the skaters legs which is a cool little detail. so it’s swings & roundabouts)
Otherwise a long lens + a flash would work too, but I do really much prefer skateboarding shot in natural light when possible.
Look at Rafael Gonzalez’ work if you’ve not done so already too, his work is beautiful!
Shooting black & white would be a good exercise too, that way you force yourself to think about how the composition & light creates contrast not the colours available in the scene.
Also note that overexposing the film will give you less contrasty files when scanned but you can pull a LOT of colour and contrast from Portra when you have a nicely exposed/flat scan.