Heart Of A Cowboy by Trish Milburn

What is she hiding? 

Of all the Brody boys, Garrett would have been voted most likely to settle down. But somehow, the right woman never came along. Until now. Veterinarian Natalie Todd left Blue Falls when she was a kid, but she seems to fit right in. Her old friends are there and she has a job with the local vet if she wants it. Every kiss tells Garrett she feels the same way he does. So what's holding her back? 

Natalie can't say, and every kiss just makes it harder. She came back to fulfill her father's dying wish, and found a happiness even greater than she remembered. The secret she carries will destroy everything, plain and simple. That's why she can't tell him. Not yet. Maybe not ever.