I have some thoughts.
When I took workshops from Dr. Greene, he said that when Plan B isn't working there are usually one (or more) of some basic causes:
1) One or both parties lack the skills necessary to engage in Plan B. At differing times, both my child and I lacked the skills we needed. I needed to practice getting down to concerns vs. solutions, for example (it wasn't working when I was focused on solutions). While kids do learn skills just by participating in Plan B, some kids need some more explicit instruction in one or more skills before they can even participate. My dd had to learn better emotional regulation skills, communication skills, and thinking skills (esp. thinking flexibly) before Plan B could work for us with regard to one particular trigger. Some kids have underlying issues that need to be addressed before Plan B can work: for example my dd has an anxiety disorder, and she really does better with Plan B when the anxiety is under control.
2) Spending too much time relying on Emergency Plan B (starting Plan B once the problem has already begun, instead of engaging in Plan B before the problem begins: so, if you've just asked her to transition and then you begin engaging in Plan B, that's emergency plan b).
3) Focusing on solutions rather than concerns ("I want to keep playing my game" might be a solution, not a concern. "I want you to go to bed now" is a solution, not a concern-a concern might be "I'm worried you'll be tired if you stay up any longer.").
4) Missing steps. There are 3 steps to doing Plan B, and it can be easy to leave one out. In particular, moving too quickly past the empathy step is pretty common-doing "drive-by empathy." It's easy to kind of go "I know you (x), but..." instead of taking the time to listen and really get down to the child's concern-and let her know you really hear her.
For me, when things were to the point that I felt like nothing I did was working, not even Plan B, despite how much effort I was putting in, when I was in tears weekly, when things were getting worse instead of better, taking my dd for an evaluation was extremely helpful. I literally had no idea that anxiety was a big problem for dd at the time we took her in. Most of her anxiety came out as irritability, wanting to control everything, tantrums--she rarely expressed anxiety in obvious ways like saying "I'm scared" or "I'm worried." It was really helpful to get someone else's professional perspective, and to have some help teaching my dd the skills she needed to do better.
In terms of transitions: this is something my dd has trouble with. She really has difficulty "shifting cognitive set," shifting gears mentally in order to go from one thing to the next. So she resists that movement. She'll be doing something, and I'll tell her it's time to do something else, and she'll just continue to do the first thing. When she was younger, she would tantrum when it came time to transistion. I found we had to experiment a lot with how to approach transitions. We did lots of warning for transitions (5 minutes, 3 minutes, 2 minutes, 1 minute...), which I still do and which helps (but sometimes when she was younger that wasn't enough). We've done written schedules (for a younger child you could do picture schedules)-my dd, with her anxiety, just loves to have things mapped out and will rigidly stick to a schedule. The written schedule could be different every day, but as long as her day's schedule is written down she feels good (the predictability helps her feel secure) and transitions are easier (she can see what's coming and when). We've used timers (to show visually how much time is left, and to remove conflict by putting the timer in charge instead of me). Different things have helped at different stages of her life.
One other thing that also helped enormously was to set aside 10-15 minutes at least 5 days a week for "special time." This was uninterrupted one-on-one time when dd chose what we would do together, and I did not ask questions, talk about behavior, or direct any play. Anything I said was positive. It made a huge difference in terms of our relationship, which made solving problems easier. It helped us nurture more positive feelings toward each other, it helped us build trust. I know it can be difficult to carve out that one-on-one time, but it's worth doing.
You'll get through this. You're not awful parents and she's not an awful kid. It'll be okay.
eta: You can ask some questions here: http://thinkkids.org/mythinkkids/
about Plan B. It's a new forum but some of the people who will reply have been doing Plan B with their kids for a long time. You may or may not find the videos at www.thinkkids.org
helpful in term so of troubleshooting plan B.