You can feel it, can't you? Right there, in the back of your mind it sits, waiting patiently, not given any attention. But on occasion it creeps up, and you notice it. It has been there for a while, like a ripe pimple hidden in the hairs of your eyebrow.
Doubt about whether it is worth it spending all that time trying to get that very last Fletching level, even though there are no useful benefits for it. Ah well, you can train Fletching while doing your homework.
Doubt about bothering with all those dailies and weeklies. They call them Distractions and Diversions for a reason. Surely they aren't trying to keep you from training the skills you actually like? Meh, who cares, this way I can still level the skills I DON'T like.
It could be that you have doubts about saving up for that item you always wanted. As with almost all items, eventually it will become obsolete, and worthless, negating all the efforts you put into it. But no, before that happens, you'll get some good mileage out of it.
Or perhaps it's doubt about keeping up with your clan duties. I'm not talking just about working in the Citadel (for the handful of clans that actually bother with it), but why WOULD you show up for wars or events that only costs you time and resources? There's a reason that TET has turned into the TMHT. No, the little time you have on RuneScape might as well be spent productively.
And why would you bother with making new friends again in RuneScape? You kept everyone on your list since the day you started playing, but over half of them don't log in anymore, and the other half keeps changing their names so you won't recognize them anymore.
Maybe, just maybe, it is doubt about all the time you spent on RuneScape and if it has all been really worth it. There might have been a time when you learned something from playing, even if it was just on the level of social interaction, but those days are gone. Or perhaps RuneScape appealed to you as a form of entertainment, but powering through the levels is still a full-time job. It might still be challenging, but even the Quests have stopped tying into one another; what used to build on an ever more intriguing world has been reduced to an occasional bedtime story.
Or it could be doubt about the future of RuneScape as a whole. One way or another, the last day of being able to play this game is approaching, and on that day, only one person will be at the top of the highscores. That person will in all likelihood not be you but you're still, in a way, fighting for that spot because that is what the game is eventually about. But you take solace in the fact that the one who does "win" RuneScape will eventually too be forgotten.
The common opinion appears to be that last day is coming sooner than later. But punching out early, especially now after so many years, would be like leaving an old dying friend. You just don't do that. No matter how much it pains you to see your favourite game slip through your fingers; you're going to sit it out till the very end, like the marathon runner that crawls over the finish line.
Some have already begun their grieving process, as evident by much repeated comments that resemble comments made by the 5 stages of grief.
Denial: "The game is fine the way it is."
Anger: "If update X isn't undone soon, I will quit!"
Bargaining: "The game could use Y, I'd subscribe if that happened."
Depression: "I miss the days when Z was still here."
Acceptance: *Player has logged out.*
Funny thing about having doubts is that, while it is impossible to ignore them forever, you can brush them aside. I think that this is the sole reason that RuneScape has been a success for as long as it has.
We continue droning on in RuneScape for the next level, to unlock the next bit of content, or get that one cape. But just when you start to get bored or have doubts, a new update arrives. The fact remains that only very small minorities quit over an update while the rest continue to play on. Just as you can put aside your doubts, so too can you put aside new content that you hate, at least for a while.