Even though I was the one that broke it off with my ex-gf (of 2.5 yrs) 2.5 months ago, I'm particularly now undergoing severe feelings of anxiety and abandonment and yearning towards our old times. Particularly at night I feel very scared and 'abandoned' and have trouble sleeping often (We lived together 2 yrs). Is this normal?

Yes, and the "what if's" it will pass with time.