Feel dissatisfied after each submission I am a third year graduate student at a "top-20" university who works on fine-grained complexity (lots of playing with 3-SUM, OV and the usual popular hardness conjectures). I have been fairly productive over the last year or so and have 3 accepted papers and two submitted papers. All of this to say that I am a fairly experienced graduate student and what I am about to describe is not anecdotal.

Every submission brings me more dissatisfaction than satisfaction. Just before I start working on a problem, me and my advisor identify a list of concrete questions that need to be answered. After lots of thinking, we have some very nice non-trivial results which gives me a lot of happiness and satisfaction. As we start to write down all of the results, inevitably, there are some more interesting variants that pop up but are much harder to make progress on. After the initial euphoria point, I feel everything seems to go downhill. There are so many variants that also need to be answered, are clearly in the purview of the problem at hand but I am not able to. By the time we submit the paper, I am so dismayed that results in the paper seem almost trivial. Perhaps this is simply tunnel vision, but I can't overcome the sadness about not being able to answer peripheral questions (although these make for a terrific conclusion section).

This has happened every single time and I am wondering if this is a common feeling. Do other people in theory community feel the same way? I am not sure if this is an academia wide feeling. My fellow graduate students from other areas are over the moon after every submission (but this is just anecdotal).

Edit - I see that there is another soft-question on the front page. I apologize for adding another one. Its holiday season and (only?) after a few drinks, one starts to ponder over these things!
Yes, it is very common. Once we spent months thinking about a problem we start to see relations so well that the solution looks trivial to us. It is not specific to academia, once we have a solution for a problem it makes the problem look easier that it really was, all the clouds that prevented us from seeing it have disappeared.

How to deal with this feeling? I haven't figured out myself but a few observations that might help:

Remind yourself what seems obvious to you is not for many others. How many people in your broad area saw the solution beforehand?

For most results (and most results are not break through results) I think it is the case that someone else in our field who is smart enough could have figured it out as well. This also has an effect that we feel we are not so special after all. Well, we are not! Once we accept that there are others who are smart enough and deal with our ego and become humbler we tend to have higher satisfaction from our contributions. Yes, X, Y, and Z could have done it as well. But we were the person who put the effort to actually do it.

Another one is to pay more attention to what problem we decide to work on. Be very clear with yourself why you want to work on a problem. Don't do so just because it looks interesting or difficult or others say it is interesting. How would knowing the solution to the problem affect our understanding and world? If we are clear about that then the fact that the solution to the problem looks simple wouldn't bother us, it would delight us instead. This might be difficult though for a student, you have to pick a problem that is meaningful and solvable. It is harder in more pure areas than applied sides. And often we don't end up solving the real meaningful problem but make some meaningful advance towards it. Keep in mind that is also valuable. Even our failures can be valuable if we can share the insights we obtained from them with others.

There is also how others judge our contribution. It is will known some areas of theory have a problem of valuing technical heavy lifting more than some meaningful contributions. Don't let them discourage you. In the long term, most of those technical heavy liftings will be forgotten, don't allow the facade to discourage you from working on meaningful problems.

Lots of young researchers focus on proving themselves. Focus instead on building good and meaningful collaborations with others to work on meaningful problems. That is much more important in the long term. With some luck you will end up with some significant meaningful contribution.