I'm currently at the point where everything I come up with either sounds too plain or like it's been done before. I feel like it's a lack of inspiration. I don't know what to do to fix this.

What do you do to get inspired? I'm at my wit's end here. This sucks.

usually if i hit brick walls, then i stop writing until something hits that i have to write down (or explode into a thousand tiny army men that take over a town...but i digress). so my advise to you is to put down the pen and paper...you're not on a deadline are you? just relax and itll come to you.
Same. Venturing into other styles of music also helps. I don't know how much theory you know, but sometimes just jumping into something new will help out.

Most of the time, trying something different is the catalyst for me. Once I do something that's different from what I usually do, I've got something good within ten minutes most of the time. It's all about experimenting. If you play mostly metal and shred stuff, and you usually use a lot of distortion, pick up the acoustic. Try new chord shapes, using unconventional shapes. Go watch Eric Johnson on YouTube playing Trail of Tears... Have you seen the weird ass shapes that guy plays?

Experiment with different time signatures, different modes, if you're into harmonies, use different intervals for it. Do something different, though.

Otherwise, don't force ideas, because the forced ideas usually aren't very good ideas. Unless you've got a deadline to have an album finished by and you just don't have enough songs, there is absolutely no reason to rush things. To me, the best ideas are usually spontaneous. You may work them out further later on, but the coolest thing is to just be playing, and then something will just click in one spur of the moment, and you'll be really excited about it.

But, most importantly, stop trying to write and start trying to learn. Whenever you learn something new, you are inclined to try it for yourself, and that leads to new ideas.
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Lol, it is almost the complete opposite of what I feel, although I don't feel any inspiration, I just write...

Yeah, try learning some more theory (if you don't) and experiment.
Maybe try researching some genres and try playing them (ones you are interested in), and see if you like what you hear...
All I can say is just WRITE. Write every single day, even if it's crap. After 50 songs of writing uninspired your 50th uninspired song is gonna be far better than your 1st uninspired song.