
Hi Marleen,
By the sound of things you're doing all the right things with your youngster, but some pups are less confident by nature and it takes a lot of time and patience to build their confidence up. Try not to be over anxious about "windows of socialisation" as the cut-off point for these is not as drastic as is generally advised - think about all the abused, nervous adult dogs who go into rescue and are eventually brought round :)
Keep working with habituating her to every day situations - don't go to too many different places, perhaps think of two that are ideal for having a steady flow of human traffic and make a concentrated effort to visit both each day on separate outings. The more familiarised she becomes with situations, the less stressed she'll be - just ensure that she's not pressurised. If people approaching her are too much for her, simply explain to them that she's shy and needs time and ask them not to try to touch or talk to her but just spend a few moments chatting to you instead. If you remain calm and relaxed, she will slowly but surely become more so too ;)
As to the vet visit, as JG advises, try and take her mind off things with treats - although if a dog is very anxious it won't eat them so this may not work. Why not pop into the vets a couple of times between now and the weekend - just to let get used to the smells, sounds etc and, if she seems up for it, the staff may stroke her and reassure her. The injection if very often not even noticed by pups so it could well prove to be something you have no need to worry about :)
Obedience training builds bonds
and confidence - so it may be worth trying to get her to do some little tricks as well as the usual come, sit, stay type of training. This needn't be restricted to when you have your puppy classes set up - at home, indoors and in the garden, if you can focus her little mind for a couple of minutes on something she has to think about and then is rewarded for you will notice her becoming braver too and can then carry these mini-lessons on to when she is away from home with you.
Try not to lose heart - you're doing well and if you can keep motivated you'll get there in the end. Best wishes, Teri :)