Do something solely for yourself:
I had a 'milestone birthday' a few years ago, and because I could afford it, and my old car needed lots of work, I bought myself a very nice car. Prior to that, I'd always had the family workhorse kind of car, while XH always had a very sporty car which was not suitable for family trips.
I really loved that car, it was bright red and gave me pleasure every time I got into it. I kept it for far too long, to be honest, as I didn't want to let it go.
Whatever your budget is, you could do something similar - get your nails done, have a new hairdo, just something because you want to do it and it will make you feel good.
Don't do anything with the presents and cards, leave them there for a few days. Hopefully in time you will feel more positive about opening them.
Make sure you book a treat for yourself though (or something else along similar lines) - my thinking in what I did was that no-one else was around to care about me that much, or otherwise couldn't afford to give me a real treat, so I did it for myself.
I'm sorry you had a bad experience some time ago. Try not to let it colour your feelings now. 