I also decline parties with alcohol!!! Sober folks have told me this may get easier, but that generally we end up simply wanting to do something else more enjoyable.
If I'm feeling like I'm "missing something" by not going to the party, I'll plan something neat to do for myself during that time. A treat like a pedicure or massage.....whatever is cool/fun for you. Invite a friend if you need accountability to ensure you follow through on rewarding yourself.
I have those down days too. I was a dry drunk for 1.5 years. For me personally, that's when the emotions I'd not processed or worked through surfaced and reared their ugly heads.....the stuff I used to drink to run from.