OK, I have some ideas for you.
- Shelties are naturally standoffish. Don't be sad about it. That's just how they are. There are some plusses to this. First of all, this means your Sheltie probably won't pick a fight with a big dog s/he can't handle. This is a very good thing in a dog park context.
- If there's a small dog or puppy pen in your dog park, use it. Most Shelties are most comfortable with other small dogs or puppies. My Sheltie is choosy about her playmates, but the dogs she gets along with best are cockapoos, Cavaliers, beagles, other Shelties, and those little white dogs. It's really adorable when she finds a dog she does like and starts playing... particularly since it is so rare.
- If your dog park doesn't have a small dog or puppy pen, go to the far end of the park, away from the other dogs. Buffy always just follows us if we go off to the end of the park. Then we play fetch with her, or if both my husband and I are there, we stand far apart and call Buffy to us and she "herds" us. I find that usually Buffy is more focused on us than the other dogs in the park.
- Look for a different dog park. I like the Glenview dog park, which is like 20 minutes away. Bit of a drive, but it's worth it to go somewhere with a nice big small dog pen. I also like walking Buffy on leash in the nearby forest preserves.
- Find a good trainer who focuses on socialization. Buffy had this and it made a huge difference. Our first class, puppy class, was mostly dogs her size and she loved playing with them. Second class was all Labs and this made her a little less engaged, but she learned how to act around Labs and other similar-sized dogs, which is all-important in this area. Basically, she learned to go up to them, do the casual sniff-sniff, wag-wag, and then keep her distance when they get crazy. To be honest I took both classes mostly for socialization... I know how to train her. (Or I thought I knew.) But it was invaluable to get help from an expert who has worked one-on-one with so many dogs.
- A good trainer will be able to recognize social cues from other dogs and teach you to do that as well. You can also read up on this online. I recommend starting by looking in the archives of Patricia McConnell's blog:
http://www.theotherendoftheleash.com/
- Obedience, agility, flyball, dog therapy.... find a job for your Sheltie. Do whichever thing appeals to you the most. But don't expect your Sheltie to get all his/her energy out at the dog park and on walks. These dogs need mental exercise as well as physical exercise or else they will spend their mental powers on finding your shoes and chewing them up or some similarly fun and amusing activity.
- Find a playmate for your Sheltie. From the dog park, from obedience classes, whatever. Just find one and set up a play date. You may feel silly about it but once you see your dogs playing together it will be worth it.
Good luck! I had this same issue with my Sheltie and others have too. Your puppy will be fine.