I used to have huge problems with this sort of thing, back when I first started roleplaying online. It took a while before I came to really "let go" with my character. I basically got to the point where I didn't want anything bad to happen to my character unless I had orchestrated it. Nothing spontaneous, no outside influence. And when such spontaneous, outsider influences happened as they inevitably will, I would get all kinds of upset and flustered, on the verge of thinking "other people" had "ruined" my character.
I've grown up a lot since then as a person and as a roleplayer, and I've learned a little bit better to just let things happen. I still struggle a bit with deliberately allowing my characters get in trouble, or going to places where trouble might happen to her. I'm still not sure how much of that is simply that I'm anti-social. The most that I do generally is allow my character to run her mouth and filter her as little as possible, particularly with regards to Rhiannon. That's just her personality though. Through time and life experience she just doesn't care anymore and doesn't bother to self-filter in 90% of cases. I'm still surprised she hasn't gotten into trouble because of it yet.
So my best advice is first, don't hold onto the character too tightly. Its a little bit like not wanting to make changes to a first draft. You have to allow yourself not to love the character too much. This will give you the emotional distance to allow :stuff: to happen, which will be what allows the character to grow and evolve and develop. Most good writing is about doing terrible things to your characters, as the author.
Second, try not to keep your character's backstory under complete wraps for too long. There's really not much point in doing a whole lot of work and preparation into a backstory and never letting it see the light of day. That's not to say you have to reveal everything right away, or even reveal everything, but rather that it shouldn't all remain blockboxed for too long. Its up to you how you want to ration out that information, in what way you want to deliver it, and who it will be revealed to. Then again, spontaneous roleplay may surprise you and force you to reveal things sooner and messier than you planned. That's where the fun begins.