Study and practice tactics. Unless you're at a Master level, just about all games are decided by tactical abiltiy (or lack thereof).
I'm not sure what you meant by "the end of the middle game"...did you mean to say you're lost when it comes to the 'end AND the middle game'?
Anyway, studying and practicing tactics will help your middle game tremendously. Again, the tactical ability (or lack thereof) of a player determines the outcome of basically all games between 'class' players (i.e. those below 'Expert' level).
There are a few good books out there that will teach you how to handle endgames. Yasser Seirawan has a book called "Winning Chess Endings", part of his excellent "Winning Chess" Series (this series also includes books on Openings, Tactics, Strategies, and Brilliancies).
Bruce Pandolfini has a book out on chess endings. You can also check out Lev Alburt's "Just the Facts", an excellent end game book.
Just about all chess instructors agree that the openings are the last thing a player should study, although learning opening PRINCIPLES is of equal importance to learning any other phase of the game.
As far back as Siegbert Tarrasch's "The Game of Chess", it was recommended to study end games first, then middle games, then openings... because it's easier to see how the different pieces work together if there are just a few of them on the board, and then these principles can be applied to the opening and middle games as well.
If you're having trouble in the middle game, it's probably because you haven't developed your planning skills yet. It's important to always have a plan, and even a bad plan is better than no plan at all. Seirawan's winning chess Strategies is a good book to study, as is Jeremy Silman's "How To Reassess Your Chess" and "The Amateur's Mind".
As for tactics, a program like CT-Art is a great tactics developer, but it may be a little advanced for someone just beginning to study tactics...
Laslo Polgar's book, Chess, has over 5,000 chess puzzles, so that's a good place to start practicing tactics... John Nunn's "Learn Chess Tactics" is an excellent tactics book as well, and Seirawan's "Winning Chess Tactics" is a great book too... I'd recommend the entire "Winning Chess" series to any player who wants to improve.
Ultimately, studying tactics will result in the most rapid improvement in your play, but it's also important to learn how to formulate a plan. Seirawan's or Silman's books are great for learning about plans, and how to develop and follow them.
Another good one-volume chess book is "Win at Chess" by Ron Curry. I use his methods when I play... I try to ask myself the questions he lists before and after every move when I'm playing.
I'd recommend any of these books to someone at your level, even if you buy them online, sight unseen. These books can help players at all 'class' levels.