Honestly, I would say that the absolute best birthday present you could give yourself would be one or two days where you take away all of the responsibilities that have been weighing on you, and spend them with friends, loved ones, or even just on your own doing things which will make you happy.
You might not think it would be that exciting, but I can guarantee you that it would be much better than any physical object, or any one "thing" which you could ask for. A couple days away is the best gift you can give yourself in a case like this.
Alternatively, maybe make sleeping your main present - spend the whole weekend in bed! It might sound silly, but I think a big part a feeling satisfied with life is making a conscious effort to prioritize the things that make you happy, rather than only prioritizing things which are 'productive' in a traditional sense.
Get takeaway, have breakfast in bed, ask for a nice and comfortable Oodie, watch movies and have snacks
You're allowed a big chunk of time dedicated to sleep!