I can relate to you and Lexi! My Mason is 8 months too and will not socialize with other dogs! I try to take him to a dog park every week, and everytime we get near the park he puts his butt on the ground and I have to pick him up to go in!
In the beginning, he was so frightened he would shake on my lap and would not move. Lately, he is starting to sniff other dogs and can play with small dogs. The funny thing is, when we're in the park Mason will not even go near big active dogs. The only dogs he would go near are small ones. But when we are taking a walk and he sees a big dog on a leash, he doesn't mind getting close to it.
If you want Lexi to socialize, it's important that you are persistent in geting Lexi around other dogs. The more she is used to being alone, the more she will resist other dogs. Although it's been a slow process, taking Mason out and seeing other dogs has helped him adjust.....I think.
Good luck!!