How do you leave the past behind

... when it keeps finding ways to get to your heart?

His past haunts me. I don't know why I do this to myself but sometimes I catch myself digging up the remains of their past relationship, trying to get a glimpse of whatever I can find that's been left behind.  I want to be honest, it hurts me to know that I'm not his first girlfriend.  In the past, I had dreams that my future boyfriend and I would be each other's first loves - that we'd learn, grow and experience everything together the first time 'round but it didn't quite happen the way I planned.  (When does it ever?) I know what we have is special but there are times when I question his choice of words, the song lyrics he sends me - it stings me to know that these exact sayings were used once before.  Is it jealousy that's taking over?  I'm convinced it's insecurity that this jealousy stems from, that I'm weaker than I thought.  I used to be stronger, more confident in myself as a person - but this relationship has proven to me that I struggle between being an understanding person and the self-absorbed girlfriend that I despise to be.  Maybe if his ex were less clingy, less dependent on him - I wouldn't feel so threatened by her presence.  I'm realizing that words aren't enough (I'm a bs-ing Communication student after all) to convince me that what I do and don't do satisfies his male ego that maybe one day he'll wake up and want someone who'd lean on him more, less independent.  He's been trying really hard to be extremely transparent with me yet I find myself holding back - I don't know why I can't just let it all out.  Sometimes I can't be myself, afraid that I'd offend him in some way, stuck in the mindset of attempting to be the perfect girl I strive to be.  (Is that so bad?)  I know I'm overthinking, making things seem more complex than it should be but it's hard.  It's the boyfriend's birthday tomorrow and his best friend wanted to carry on the tradition and surprise him - so I (along with my best friend whom I personally invited - she's really the best for supporting me) will be meeting his ex-gf for the very first time.  I'm not ready for this but I'm not sure when I'll be - it is his day after all, and maybe - just maybe - my insecurities will subside when I meet her.  He's been frustrated by my stubborness, knowing that something's been bothering me but I refuse to tell him.  I dislike hiding from him but if I say anything, his surprise will be ruined.  I just have to keep convincing myself that I can get through this, and I will, I hope.