What to study after someone stopped playing for years? I stopped playing for 2 years and then I had some periods where I played and studied chess, but never for more than a few months in a row. I have an ELO of 1900.

After these 2 years without playing, I noticed that I take way more time to think during my games (did I lose confidence?), sometimes hesitating for a long time between two good moves that should not matter that much (like "should I play Bd3 or Bc4 in the opening?", or "should I take with the c or e pawn?").
I also noticed that I forgot some opening lines (which I think is completely normal) and some endgames too.
I almost always end up with time trouble, even though I read guides about time management.

My question is: what should I study to get back to my level before I stopped playing?

I used to do a 15 min tactics on ChessTempo and Lichess, but I stopped because in a game you never know when there's a combination and the puzzles on ChessTempo became so difficult that I often ended up playing the moves I thought was best because I could not find the combination of +10 moves in my allocated time frame.
Now, I do the exercises from Practical Chess Exercises which are more similar to what you have to do during an actual game. I also do the very easy exercises in Ct-Art 6 because I think it takes me too much time to find these in a real game.
I also started to review my endgames and I do endgames exercises on Chessable.

For the openings, I tried to switch to new openings, but it did not go well, probably because I needed more time to learn them.
So, what I do now is I review my old openings.
I'm not sure I should review my openings though, because I'm pretty sure I don't lose my games in the opening.

Is there anything else I should do?
Should I stop review my openings and focus on other weaknesses?

Edit: By the way, I did play a few tournaments (not as much as before I stopped playing, but still) and in a club since I stopped playing.
Having had experience with this exact problem myself a while back, there is only one advice that I can give: play as much as possible! If the time management issues you're describing are a new occurrence they are most likely due to you being rusty, and they should be corrected by regular tournament play. It takes time to reinstate the trust you had in your own in-game judgement after a long period of inactivity, but it will most likely come back if you keep on playing regularly.

As a final note: studying chess theory is never a bad idea if you're looking to improve, but don't expect it to fix your problems with time management.