Tradeskill abilitiesFollow

You want to primarily use your newest abilities. These are the abilities that will increase your profession skills (metal working, scribing, artistry, etc.) the quickest and they will also have the greatest effect on your recipe turnout. Look at the descriptions for the abilities. Some tiers (levels) will primarily deal with increasing your durability and others will focus on increasing your progress.

I generally keep 6 counters/abilities on my tradeskilling hotbar: Your newest set of counters (the 3 you most recently got) and the previous 3 (from the last tier). this will give you one counter of every "type" and will give you the flexibility of using either progress or durability skills as you see fit. Anything beyond that you can safely ignore and never use again (in your example, you can just forget about your L2 skills from here on out).