My first serious bf (first love, first everything- I was 17) went off with my best friend, and I was the last to know. The worst thing was the humiliation of people feeling sorry for me. I can still cry when I read my diary from that time- I really thought I would just die of unhappiness. I'd lost my best friend and my boyfriend in one fell swoop, and I was just about to go off to uni in a strange city where I knew noone.
My new flatmates were wonderful- 1 is still my bestest friend in the whole world. Uni was a great experience. I made up with my ex best friend, because I didn't want him thinking he was important enough to wreck a long-term friendship. Once I gave her my blessing, the attraction wore off pretty quickly. She cheated on him and treated him like crap, so that he ended up crying on my shoulder at a party that she didn't love him and he'd made a terrible mistake (I was with new boyfriend by then, and despite myself felt nothing but sorry for my ex) I feel proud that I behaved with dignity through it all, and was the bigger person. It really made me stronger. The last time I saw him, I was actually running the half-marathon (and dating DH, who was everything ex aspired to be- haha! Ex was a small guy with a complex!) and I beat his time. I now remember him as a bit of a saddo! But I wish I could have told that heartbroken 127-yr-old me how things would turn out. Incidentally, best friend later fell madly in love with a bloke who did the dirty on her, and she was devastated (despite dismissing my pain as "alls fair in love and war" at the time of her deception!) I am a strong believer in karma!!
My next bloke was lovely, and still is! We were together nearly 7 years, and I felt we needed to decide where we were going. He was a bit of a perpetual teenager, and commitment-shy. I ended it, and felt ok. I knew I wanted marriage and a family one day, and he couldn't even give me "one day". He was gutted, but we parted amicably and have mutual friends. He is still unmarried to his new gf, who he met before I met DH, while I have a wonderful DH and 3 kids.
I really feel that both these experiences have made me a far stronger person. I am not on bad terms with any of my exes really, although I was hurt at the time. I'm a big believer in looking to the future and moving on, and I know whatever happens I will cope. I know I've not really had a really really bad experience, though, and I count myself lucky.