Fight your urges to deny or reject nice things. Fake it til you make it.
My parents were the same. Presents were used as blackmail and taken away for punishment so nothing i ever owned felt like mine. My dad constantly reminded me that I was living in HIS home, not mine.
I had a really weird thing about birthdays where I tried to hide it from everyone because birthdays made me feel dreadful ever since a child. My best friend and I turned it into an inside joke where we lie to everyone and swapped birthday days (we're same month, different days). Now I get birthday wishes and the occasional gift on her date and I associate it more with happiness since they fell for our stupid game.
You don't have to copy that, but you and your girlfriend should find your own ways to twist awful memories and overwrite them with something new to make it enjoyable for you. It's all about taking a bad association and adding a new component so instead of your brain immediately thinking on the past you have a new door to walk through that isn't the depression.
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