I know what you mean 100%. I was with a girl for 7 months and she suddenly dumped me without warning and started dating someone that everyone considered a scumbag...and I couldn't even be mad at her...all i could do was hurt. But that's part of life...you have to make risks to have anything worthwhile. I understand you're hurting (and i have felt that physical pain that you mentioned), but I stuck with it and picked up the pieces and moved on...it's been two years since then, and for the last year I've been with someone else and my life has never been better. I know that if something happens with this girl that I'm with now it will hurt, but if I hold back, then I KNOW that it won't work out.
Remember, You're braver than you believe, and stronger than you seem, and smarter than you think.
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You only live once...but if you work it right, once is enough.