You mention "I am 25 years old... Am I too old to start chess".
Emphatically, no. You can resume playing at that age and continue to improve your skills. Chess is not a physical skill; as others have stated, it requires mental training.
I cannot find a page currently due to network blocks, but I would suggest learning the so-called 10 Commandments of chess.
1. Do not move a piece twice when opening (the idea being it is better to develop two pieces that move one twice)
2. Move to control the center of the board.
3. Knights before bishops. I struggle with this one and it is likely a tactical blind spot on my part (we all need to continue learning). I think that it relates to the knights being more versatile when controlling the center, which plays into 2.
4. Trade when ahead. If you ever are up in material (knowing this as stated by other answers is critical), continue to offer trades. If your opponent takes them, you push your advantage. The opponent who refuses must generally spend a move to get away and thus is not able to advance a plan.
I cannot recall the others, but initiative is also key and is an underlying backbone to much of this. If you retain initiative and freedom of movement, you are more likely to succeed. Constantly having to react to an opponents attacks is generally a disadvantage.
As for ratings, once past the preliminary period (as mentioned by others) it becomes increasingly difficult to change your rating. To increase it you must play (and for maximum effect, beat) players who are more advanced than you are. Winning will require you to learn new skills and tactics.
"Whenever I lose my game, I lose my confidence and very afraid to play next game and I feel bad sometimes, especially if I lose against a lesser rated player than me. Does anyone get this feeling?" Yes, I would venture to guess that it is fairly common, but remember, the ratings are not exact and everyone has good and bad games. Don't let yourself become fixated on your (or your opponent's) rating. Learn and practice basic strategy and tactics to improve your gameplay.
Two things in closing. One, I find it extremely difficult to focus on an electronic board, to the point that playing online I will often set up a physical board and move the pieces on it to keep perspective. Two, simple algebraic notation (a-h, 1-8) will help you document your games so that you can look over them later, figure out where things turned for the win or went off the rails. This can help a coach look over a game and suggest changes and help you learn more about how you play.