我的朋友要请我参加她的生日会而我不能去，该如何用英语回信拒绝？

Dear Tina：
I am grateful to you to invite me to your birthday party, I also want to, but recently my body is not too uncomfortable, so I can not come, but Happy birthday to you! I hope you can forgive me!
Yours Kerry

（很感谢你能请我参加你的生日派对，我也十分想来，但是最近我的身体有点不太舒服，所以没法过来，但是仍然祝你生日快乐！希望你能原谅！）

Dear Tina,

Thank you for inviting me to attend your birthday party,I'd like to.But unfortunately I don't feel well recently,so I'm sorry to say that I cannot attend your party.But please allow me to say Happy Birthday in advance.

Yours,

Kerry

这样好，feel good 是指心情好，而feel well 才指身体好。
先感谢她邀请你去，才阐明不能去的理由并致以歉意，最后提前祝她生日快乐
这样她就不会怪你的啦 ：）

Dear Tina,
I'm pleased to be invited to your birthday party,but I'm afraid I can't go there.I haven't been feeling well since last night.Maybe I have a fever.I'm so sorry that I have to go to the doctor's and can't go to your party.Please don't worry about me.I believe you'll have a wonderful party and I'll be well soon.
Yours Kerry
呼呼，临时写的~应该没有任何语法错误。。

月，日，年

Dear Tina：
Hi, happy birthday!
Thank you for you invited me to your birthday party, I'd love to. But today i am feel not good,maybe i was ill.so i'm so sorry, i can't go to your party. very sorry.
have a nice party time.

yours Kerry

It's very kind of you to invited me toyour birthday party .But unfortunately,I'm not feeling well at present. So I'm sorry to tell you that I couldn't go to your party .