Flat beer probably means you didn't add enough sugar, or didn't cap the bottles properly. Sour beer likely is an infection. Gushers are definitely an infection.
I had a string of infections in the past that almost put me off the hobby. They went away after I pitched all my tubing and any plastic that smelled, then switched over to glass carboys for fermentation. I'm not saying you have to go that far, but you've probably got some piece of plastic or tubing that is contaminating the beer as you bottle it. I'd advise you to replace any plastic that is discolored or smells. Then the next time you use your equipment, clean everything with hot water immediately after use, sanitize it with Star San (I use a spray bottle), let it air dry overnight, and put it away the next day. I store all of my siphoning equipment in trash bags.
Infections suck and it can take a while to track down the source. Just remember that any equipment that comes into contact with the wort after the boil must be clean, clean, clean. You can't clean it too much. And Star San is a lifesaver.