Questions after playing first tournament

Hi, first of all, MANY THANKS for providing people with guidance for the past years, it has definitely helped me get to where I am, and recently I participated in a tournament to get a ranking (It's 1109, I don't think that's bad for my first), and I wasn't as nervous as I thought I would be. I learned a lot, but the main thing I learned is to not take my skills for granted. I played against a pen holder with MUCH less knowledge of techniques (everyone basically beat him), yet he managed to beat me because I took the game for granted and I thought that it would be an easy game. Anyways, onto the questions:

- I lift my stroke too high when I counter with my forehand or backhand with topspin. Obviously, lowering the finishing position makes the shot go perfectly on the table, with more than enough topspin, but when the finishing position is above the net from my perspective, the ball goes way too high. Is there anything that I can do to help lower it and maintain it low and consistent?

- I seem to get "stuck" sometimes when I start using my offensive tactics. Basically the scenario goes like this: I serve sidespin/underspin to opponents right side (I'm a lefty) and I automatically get into offensive position to attack. As soon as they return, depending on how the ball comes or where it goes, I either topspin the ball or just hit a strong forehand with slight topspin to the side where I see the opponent is weakest at, or very far to the sides of the table. However, the problem is when they return it, and it goes towards the center of my body, or towards my elbow. I just seem to get stuck watching the ball and not moving to counter it back.

- Another problem I see having is to maintain fast and consistent in a match with a person that is very high ranked (Over 2000). They counter the ball very quickly, and usually when I respond I either get stuck, perform the movement incorrectly, get tired way too quickly, I don't get there in time, and when I do hit it back, it's with weak spin or it's too high. Although it isn't "very common" in my game, I start seeing a pattern where usually when I play against these higher ranked players, these are the things that prevent me from getting the point. I'm starting to get my ground, though, I lose against these players 3-2 and rarely 3-1, compared to constantly losing 3-0 a month or so ago.

Ive been talking to other players, and when they see me play, they usually advise me to play more "relaxed." I don't really know how, since I don't see myself, but they tell me I play with my back too straight sometimes and I play too "still," even though I have my knees bent. How do I get my back more "curved-like," since I guess that's how it's supposed to be..? 

I probably have more questions, but it's pretty late, I'm tired, and these are the things that I mainly noticed in my day today. Thank you very much!