Sorry to hear your first time soldering copper pipe went badly. It isn't hard but it does require some practice to get a feel for it. Resoldering bad joints is possible but not simple. You have to first heat and separate the joint then heat and wipe off excess solder. Then you may still have to sand off some remaining solder to get the fittings back together. If you are set on staying with copper then your best bet would be to cut out and replace all the affected copper pipes instead of trying to repair the bad joints. Otherwise I suggest you get a roll of PEX pipe, fittings, and a crimp tool. Sharkbite fittings can be used to join old copper to PEX.
PEX is really the way to go. Copper is so expensive and doesn't bend around obstacles. PEX comes in 100' rolls and can bend and slide through holes and around corners. Soooo much easier to work with.
Drain line and Reheat with torch. Remove and clean with new fittings. Brush fittings and use a nice even layer of flux then heat up and add solder.
If your done with soldering altogether for what you would pay a plumber to do it you can buy new supplies and do it over again but rent a pro-press tool and get press fittings. Press it all and you don’t need to solder again.