Sometimes when I loose motivation I start to watch shows tv and play a game to try and find something new to get into or, I look into old fandoms I really liked. Twitter also helps inspire me a bit! And in my spare time I look around on here for ideas I also started making a list of them for when they pop into my head and so I can remember them all!
I hope this helps!
Even though you want to roleplay, don't force it. Breaks are often good. I'm not sure how long you've been roleplaying but if it's been a constant thing for the past months or years, it's hard to keep motivation like that. Take a break. Get together some characters and ideas. That's what I do. Though everyone has their own thing. Listen to some music, maybe do solo stories with your characters, watch movies or anime play games if that's your thing. Just don't force it.
Ask yourself “Why are you RPing to begin with?”. There’s a reason and if you find it you’ll be more invested.
I also stopped caring about making the perfect post and just write. Some days my post will be to my satisfaction and other days they’re cringe but I’ll still send them through. I can’t be 100 with my posts every time. It’s not realistic.
Also, if you’re stuck on an idea talk it out with your RP partner. You’re not the only person in the RP so talk things out!
I personally believe this often comes down to settling good habits. Like exercise, any effort-requiring activity will naturally become easier if you do it regularly, especially if you reasonably push yourself a bit (and I stress, a bit) beyond your comfort zone. Run a bit every day and you'll find you'll get more used to running and it will not seem as daunting. But if you start skipping days and slacking off, the task can seem monumentally harder as one degenerates out of the habit of putting in that work/effort. Likewise relying excessively on being on the right mood to write makes it increasingly harder to get in that mood in the first place, as the effort seems harder to slide into.
In short, I think it's a good idea to continue to write regularly. It doesn't have to be necessarily writing for that one RP if you're tired of it, or even RP at all, but keeping a regular schedule in which you just churn out some words could be good for keeping the mindset from slipping.
That being said, that is more of a long term solution, and there are other strategies and advise I personally use in the short term. One of them has pretty much already been mentioned, I go back to the sources of hype and inspiration that led to create the plot, characters, world, in an attempt to recover some of that initial momentum. I may also just talk to my partners or read back the previous posts in the RP to gain interest in what's to come or simply wish to continue out of appreciation for story crafted until that point.
One could also try to approach this from a different angle altogether. One of the ways I write even if I'm not especially motivated is to systematize my approach somewhat. I break down my coming post into smaller problems and segments, figuring out what I need and want to happen in each. Smaller, more concrete problems are less daunting to approach than a big and vague one, and completing those pieces can give one a sense of progress and accomplishment that helps to go through the rest of the post.
I suppose I am posting this a bit late compared to when the question was asked, but nonetheless, here's hoping it proves useful to somebody who comes across it.