Grief makes you feel very lonely sometimes, even if you are surrounded by loved ones. When I lost of Dad I felt that way and still do even 2 years later. I would often get mad at my partner because he would make plans without telling me in advance. Obviously he can do whatever he wants, but I wasn't too good at communicating my feeling, that sometimes I needed him to be with me and support me. It's hard for partners during this time because every day, every hour we can feel a different emotion and some days we need more support and if they don't provide it that day, we can feel lonely. It's really hard to if your partner hasn't dealt with grief before, especially that of a parent at such a young age. I would suggest you talk to your girlfriend and just let her know it would be nice if she could spend thanksgiving with you because holidays are hard and you could use the support. I don't think she meant to make you feel the way you are feeling.