Yeah, knowing the truth about yourself is definitely a burden in a situation like this.
I was blissfully ignorant about my AS when I met my boyfriend, and didn't think much about disclosing it to him after I received my diagnosis, since I was coming to terms with it myself. He has been receptive, and accepted it as a part of me that has always been there.
It sounds like you may have been in a similar situation with being diagnosed while you knew your girlfriend, except you were both friends at the time. I wouldn't be able to keep it quiet for that long, more power to you.
But still, honesty is the best policy in a relationship. Since you know about this now, think of it as a way for you both to know how to work on it to make things better. If one or both of you does not know the reason for your strange behaviours, then there is more misunderstandings and less of a chance to help.
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Your Aspie score: 93 of 200
Your neurotypical (non-autistic) score: 109 of 200
You seem to have both Aspie and neurotypical traits