Hey! It sounds like you haven't been at this for very long... maybe you have and I just got the wrong impression. But I'd like to share with you some ideas that might help your attitude.
First of all, someone's birthday should never be a reason for you to feel miserable. Yes, there are going to be some food challenges, and that cake to deal with, but this is just a small blip in your journey. You do not want to be a downer on your boyfriend's birthday! So, you can enjoy the time with him and just make sure to adjust your approach to the food.
- You can have some cake, if you choose to. Think in terms of cubic inches, and cut yourself a thin slice. If it's a round cake, go for an inch wide at the edge. This is not going to do terrible damage unless you let it! NO second piece--and in fact, if your boyfriend doesn't want to take it home with him, or if you live together, you'll have to freeze him a chunk and throw the rest away. Yes, I'm serious.
- You can eat at the restaurant. Skip alcohol. Most places these days can do a grilled (not breaded, not fried) chicken breast sliced over salad with dressing on the side. There you go! The idea is, find the items that will do the least damage, and eat a reasonable portion. Leave the leftovers on the plate! You don't need them.
Second (yeah, this is a novel...
), if you are exercising to the point where you are that sore the next day, then you need to take things easier. If you're new to exercise, this is very important. You don't want to hurt yourself while you're changing your exercise habits. No need to go for a boot-camp style breakdown! And RealCdn is right about rest days--you need them!
Third, please please let go of the idea that if you don't do everything perfectly, then it's all hopeless! Many people make this mistake. It's not an either-or situation you're in--it's a situation where you can control what you do, within limits, and sometimes you'll go over the limits a bit. Just keep trying to keep those to a minimum and stay on your plan as much as possible.
And finally (I know, everyone is asleep by now), make sure that the plan you're following is reasonable--that you're not trying to get by on too little to eat while exercising some massive amount. This is another recipe for giving up! Moderation is the best way to go...
Cheer up! You are doing fine!