It is never too late to train your dog. The saying "you can't teach an old dog new tricks" is the biggest lie there is. Keep in mind that all puppies learn at different rates and as a puppy their attention spans are very short. Here is a tip on getting her to sit for you all the time... Everytime she wants something make her sit first before you give it to her. For example, she wants to go for a walk have her sit before you go outside or she wants a toy, have her sit for it and reward her with the toy. She will figure it out that she has to work for you to get what she wants. Rember to keep it fun, all my dogs love doing their tricks and yours should too. Also, keep the practice lessons short, 5 to 10 minutes a few times a day. Any longer than that and they will become bored and won't do a thing you want them to. Good luck with training!!
