Good lord, calm down people. While I don't think an uncontrolled horse should be bred either, there are nicer ways of telling people your opinion. You don't need to jump down the OPs throat or harass her. And before someone says they are just offering advice or whatever and not being rude, please don't bother. I agree with 90% of you, I just think many of you are being a bit harsh.
ANYWAY, I'm in the same situation as you. My mare is quite a handful- about 50% of the time. The other half she's a perfect angel. All hell just breaks loose on her bad days and we both end up frustrated. I'm having my old trainer come ride her on Sunday as she's ridden her a few times before and could at least handle her a little better than me when she acts up. I've been contemplating selling her to someone who has more patience than I do, but like you I'd be heartbroken to have to get rid of her so I'm opting to get more professional help.
When and how does she act up? Is it on the ground, or when riding? Does she pull away from you when you lead her, or does she get in your space? Does she buck/rear when you ride her? Is she better off the farm or in a different environment? These things are very important. The first step would be have a vet check her out for any illnesses or injuries. If she's worse when you're getting ready to ride and when you actually ride, she may be sore and knows that work=pain. That can cause a ton of behavior issues. Even professionally fit tack can cause pain, so check that all out too. I admittedly don't know as much about illnesses that can cause severely bad behavior as I probably should, but I know lymes is very common and can present it's symptoms is many ways, including behavior problems.
After you've ruled out any physical reasons for the issues, get an EXPERIENCED TRAINER. Your best friend may be fantastic around horses herself, but it's a lot different than finding the errors in your handling and being able to explain how to fix them. When Roxy had problems loading, my trainer noticed I dropped my hand about an inch when I wanted her to go forward; that small movement alone that most people wouldn't think twice about was enough to make her stop loading regardless of any other influence. Since correcting that, as well as a few other very minor handling errors, she self loads perfectly.
You need to be confident and send out the right vibes to your horse. If you're nervous or unsure, your horse WILL take advantage of that and you're never going to be able to fix the problems. If that's the case, you're going to need to either drastically change yourself, or sell her. I know with my mare I am perfectly capable of handling her, I just get frustrated and need a little help sometimes. But I'm confident and determined to make her ALWAYS realize that while she may be a lot bigger than me, I'm in charge. If you don't feel you can do that with your mare, you're never going to be successful.
Also, as others have mentioned hormonal supplements may help. But speaking from past experience, they definitely don't usually make a difference with every horse. Roxy has been on practically every supplement in all the local tack stores and the only thing that helps at all is Perfect Prep, which she gets before competitions to help take the edge off. My sister's gelding is a bit spooky and has a few problems, so he's on mare magic. It's worked wonders for him and he's a totally different horse, however he also has chronic lymes and treating that to help bring the levels down played a big part in his change too. I've not found mare magic to make a bit of difference with my mare. Her problems aren't hormonal, it's just her personality. She didn't inherit the behaviors from her bloodlines, and she was just as bad with her old owner and every trainer I've ever taken her too. She can be a pain and she definitely isn't for everyone, but for those that can control her she's a lot of fun... I'd suggest writing down how each day goes with her too to see if there's any quarilations (sp?) with her heat cycles. Depo shots can make a big difference in some mares, though I noticed no changes in my mare.
If you can give more specifics I'd love to try to help you, good luck!
Worth The Wait <3