Hi guys! HELP!!! I just rescued a little 4 lb girl about 1 week ago. The rescue told me she was approx 2.........but, I took her to my vet and he said she is probably more like 5 or 6

! Needless to say, I was a little shocked. There is quite a difference between 2 and 6.
Anyway, she is just the sweetest thing, but has obviously been used for breeding.

Now I am trying to housebreak her and she really does not seem to be catching on at all. NOT AT ALL. She is skittish and "dribbles" everytime she gets excited and thinks she is getting picked up. Could the "dribbling" be a medical issue related to excessive breeding?
I know training takes time, as I have had to be patient with my other girl, but I am concerned that at the age she is probably at, and her tiny size..she may never be able to be trained. I am currently keeping her crated at night only..and I feel bad about it because she has probably spent a lot of her life confined. I just don't know what to do. She is also become EXTREMELY attatched to me....I MEAN SHE NEVER EVER LEAVES MY SIDE...EVER. My instinct is to pick her up and cuddle her when ever she wants it, but I don't think it is good for her or my other girl, who is now getting a little left out because the new one is so demanding of my attention.
I am sorry to keep rambling on, but I would love advice from anyone who has a rescue and gone through this. I just adore her, but I do have my hands full at the moment!