Just because a piece or square is defended doesn't mean one can't or shouldn't "target" the piece or square. For instance, say your knight is on a particularly nice square that I covet... it's defended by a pawn. If I take with a bishop(or knight etc.) then you take with a pawn and I take back with my knight....as long as my knight is in the position I deem necessary for future endeavors and I can defend it adequately? That's a good exchange and I'm up a pawn.
If you defend a piece(lets say a knight) with a bishop and your other knight and I attack with my knight, bishop and rook..... then my knight takes your knight, you take back with your bishop, I take back with my bishop, you take with your knight and I finally take with my rook? I traded a bishop and a knight for your two knights and a bishop. I'm up. Especially if that results in a superior position for me. Think of it as escalating attack/defense. You defend with a piece, I attack with a piece, you defend with another piece, I attack with another piece, etc. Eventually, either a massive bloodletting occurs or De-escaltion where we both start looking elsewhere for targets and slowly move pieces to other areas of contention. Brinksmanship.
Sometimes it's advantageous to trade for a lesser value. Say I'm up a piece but your king is defended adequately behind two pawns. It might be good for me to trade my knight for the two pawns covering your king. At the end instead of being up a piece(3 points) I'd only be up two pawns. But those were critical pawns in defense of your king. I've been known to take a pawn in exchange for a bishop if it breaks down the kings defenses and allows me access to a mate solution. I'm not saying trade your pieces or make sacrifices willy nilly. Have a PLAN and be DAMNED sure the plan is sound before making any serious moves like that. Work it out in your head. Hard to do at first, and you will make mistakes. Grand Masters miscalculate. Just pick yourself up and get back into the fight... :)
I try like hell not to sacrifice or trade pieces unless it's absolutely necessary to my plan to better my position or pave the way for a mating chance. Others will trade every chance they get. I don't let it bother me as long as it's an even trade or I'm up at the end. But I don't go looking to trade off pieces unless it's part of a plan. A plan to mate the king, a plan to trap the queen, or a plan to take possession of a square I really covet etc. If you run into one of these type of players, try to get the best trade possible and play on :)
One final thought. In your last game you were REALLY concerned over what bishop was “bad”. I personally don't look at pieces as bad or good. I understand the concept. I just don't embrace it. Perhaps I will as I improve as a player.... For now I suggest you try to develop your pieces to good squares and don't even think about good/bad bishops. Their all good in the hood :) If one is trapped behind a pawn formation, eventually that formation will change and.... surprise you have a bishop that can now enter the fray. And yes, it's not doing a ton of good in the meantime but it IS backing up those pawns it's lurking behind. So it's not as useless as some might make out....
Good luck in your next contest. Have fun man. Peace
I am a tactical player by default,I know next to nothing,positionally.