Feel dissatisfied after each submission
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.