I feel like I can't come up with anything anymore. Lately I feel like I constantly ruin all my riffs (Making something up to follow a cool idea I had, but it sucks and I can't get it away from it, so I have to scrap the whole idea).

What really struck me as disheartening is that I can't even seem to come up with a decent chord progression. Everything just sounds weak.

I don't know what's going on. Any advice?
"This nightmare's gonna break me.
Please, Daylight, save me..."
Quote by Hail
listen to new genres

This. Listen to as much music as you can in general.

Also, look up some common chord progressions for inspiration.
Had the same thing happen to me for about a year - what solved it in my case was a breakup, a college writing course, and forcing myself to follow through, even on ideas that I thought weren't top quality. Eventually I started coming up with new stuff again - it took me about 3 months to go from nothing to writing a full song every couple of days. I also find recording everything really useful - a dumb riff on its own is still dumb but a dumb riff with another dumb riff intertwining and providing counterpoint is great, and jamming over my own recorded noodling is the only way I've been able to figure those parts out.

Be prepared to really struggle for a while (for me it was 4 months but it might be anywhere from a few weeks to 6 months of really hard work and forcing bad writing) to get back on your feet but when you start writing good things again it'll be worth it - and you'll be able to recognize better what's good.
Also try jamming with other people, especially those you don't play with often, they will never play or listen to exactly what you do thus giving you plenty of material to try out.
DONT EVER THROW ANYTHING AWAY... sorry to put that in caps but really. I am going to make you one promise, in about 4 weeks you are thinking, gosh i wish i had that riff somewhere because i dont know how i played it exactly, and well, its down the bin at that time. You dont want that kinda situation. I've been there several times
When we become preoccupied with not creating the same thing again, we can't help but create the same thing again. Let your ideas flow freely and ya should have the good stuff flowing again soon.