The reason some of your spokes get so loose is because they were too loose in the first place.
If for whatever reason you still can't make your rear wheel durable despite it being built properly (unlikely, but possible if you're quite heavy), it can be made dramatically stronger by reducing the dish. If it has a 130mm spacing you may be able to push it to 135 by spreading the dropouts (my old Giant CFR1 was apparently actually made to take a 135mm spindle, which was nice - makes me wonder why road bikes don't adopt yet another MTB standard), or by installing a shorter cassette body and losing a cog or two.
You should never again be confused about which way to turn a nut or bolt unless you don't know whether it's a right or left-handed thread, once you've seen this:
Given the familiarity people tend to have with their hands, you shouldn't even need to actually point your thumb and curl your fingers; just do it in your head. Use your left hand for left-handed threads.