The best thing you can do is [b]SAY SOMETHING[/b].

It doesn't matter if you're a GM, a player or in a one on one.

You need to tell the people you're with that you're not feeling it and you know what...they probably have a feeling you're not feeling it either. So don't play chicken with people who are just coasting along without anymore interest or passion. That is a waste of everyone's time and it makes the problem worse. In order to fix your problem, people need to know about it and that way a solution can be made.

Sure, sometimes you're just at the end of the rope but for the love of all things responsible and respectable, don't walk away without trying to end things properly. 

Don't be that guy who says they're leaving and then leaves. To me that spells laziness and cowardice. We generally make statements (at least I see a lot of them in threads) that underline the need to speak up when you're no longer motivated. People need to communicate, they're encouraging it but it still needs to happen. When you sign up for a RP, you're committing yourself to sticking with it and if you can't anymore and fixing it or adjusting it won't work and you've tried, [b]THEN[/b] walking away would be suitable. But do not just leave or keep quiet and hope you feel it in a few days time because let's face it, that rarely happens.

If people know someone isn't fixing it, they can add something to the RP or talk about what the issue is. Maybe your character sucks and it's your fault you've made that can't do anything with them. That's partly a GM's fault for allowing it but it's also on you. Maybe your real life is bogging you down, but then you need to say something and work with people to make sure you don't fall behind and wreck the scene, or you need to write you character so they're off to the side so you can continue later on. Or maybe you're not interested in giant robots anymore, but you need to find a way to get back into the zone. Watch youtube, talk to your RP buddies in the OOC, drink (if you're of age...) or whatever. But do not, do not just give up.

Becoming complacent is infectious and it doesn't go away.

You need to treat your disinterest like any other issue, get help.

Remind yourself why you joined, re-read the IC and don't give up. And by that, I mean talk to people and brainstorm ideas. If you are miserable and clearly dragging the story down, you should say something if no one else does. But don't walk away without doing something or that reputation will stick with people. Always end things on good terms if you can help it and own up to how you feel and what's going on in your head and why words just aren't coming out of those fingers of yours.