It's simple: you thought you were good, and maybe that was true compared to other silver leaguers.
But now that you know more about the game and see your opponents and teammates performing better, you started to realize your own mistakes, which doesn't happen for 90% of the players below plat.
But yeah, the only key to improving is to practice a lot, generally last hitting and some champion-specific mechanics with your mains.
http://leagueoflegends.wikia.com/wiki/Elo_rating_system
This page pretty much explains it in depth. Take a look at the line:
"Elo changes are higher for your first ~50 games ranging from 50>13"
No - just compare your level vs the level of other in lane after the initial clear. Also, in the situation you described, your laner is probably heading back from base, and the lane is either frozen on the middle or the enemy laner is pushing it, so it's not much minion xp or gold lost for him. Chances are the enemy is low (after the trade that caused your laner to go back or die), so with your teammate and you there he is either forced to back, or you can initiate a fight.
Maybe the best start to improving is trying to keep track and remember what you do, for example, experiment with the most common jungle routes, watch and remember the exact time you arrive at camps, know the speed of different junglers. At your elo, many junglers follow the same route, creating the potential of counterjungling if you know where they are at any moment. This way you can predict where they are going to gank, allowing you to countergank by following them or laneganking.
Another tip: ward or have your midlaner ward their wraiths after the initial clear, this way you will instantly know where they are and where they are heading.
Be careful with invading when they have stronger top and mid presence, however, when your mid and top are ahead/stronger in that phase of the game/their opponent is low, go ahead and take at least their wraiths and pressure them. Always coordinate with your mid or top laners though.
Either way, you are at a good elo to practice jungling, very few people main jungle there and when you are comfortable with the basics, you can jungle with pretty much any jungler.
