Looking at the pictures I'd say the prep work was lacking more than your staining ability. It looks like you didn't get the glue off the boards before you applied the stain. Some type of a sanding sealer will help with even application of stain. I have long since ditched the brush when applying stain and dip a piece of old terry cloth rag in the stain and work it into the wood grain. Let it soak for a few minutes and wipe it off. Wipe it off before it starts to dry or you will have a mess. As others have said pine is hard to get a uniform color when staining.
Wood with no finish or protection will out last the kids, grand kids and great grand kids if kept indoors. I have found boards in my father in law's barn that were likely 70+ years old that were as good as one cut yesterday.