D300s + 80-200mm 2.8f

[genuinesqueak]

Hi there,
I feel a bit silly asking, but I have looked around and haven't found anything that really answers my questions.
I recently bought a second hand 80-200mm f2.8 lens. To use with my D300s for horse sports photos, and especially in dull light situations. Anyway I obviously haven't ever really used anything as nice as this, my 50mm f1.8 is about as nice as my lenses go so I'm having trouble actually getting decent pictures with it. I knew I'd have trouble, because I've been a bit "out-of-action" for over a year now as I had a baby and for some reason being pregnant and having a baby turns my brain to mush, I lose most of my enthusiasm too. I feel like a complete beginner again! I never used to be this bad, two years ago my photos were published in a national horse magazine. Now I'd be too embarrassed! haha
I've always used a fast shutter speed when taking my horsey photos, I found they'd be crisp and bright, and for some reason I just knew how to do it that way. I've gotten some OK ones now but my dump rate is higher than normal...
I guess I just want to know where I'm going wrong so that I can fix it and practise.
I'll post a couple photos. The horse one was one of the first lot of photos taken, and the only one out of them I like. I haven't edited it other than resizing. The puppy was after I changed settings, again not edited other than resizing.
Hopefully the pictures work. Anyway thanks for any advice!
(BTW Since being resized they seem to be sharper and look better than the original pictures, is this just some illusion?)