This is such a common issue with gentle parenting. I am a huge fan of it as a process/theory but it sounds like you need some troubleshooting rather than more methods.
Were you attracted to the idea of gentle parenting in the first place because it promises low/no conflict and/or because you find the idea of punishments and rules and being an authority figure a bit scary and unpleasant, and hoped to have a fully co-operative approach?
Are you in general conflict-averse, e.g. anxious in the presence of arguments or disagreements?
Do you have trouble setting and enforcing boundaries (in a parenting context or the context of any relationship - do you have people-pleasing tendencies?
Is it very difficult for you to cope with your child being upset - would you avoid trying to upset her whenever possible?
If so - it's very normal to end up in this situation where you are trying your hardest with all the gentle suggestions and then when they go nowhere you lose it, shout, say things you don't want to, maybe even act physically rougher than you want to. You know that's not gentle so you beat yourself up thinking "if I can just get gentle right it will work" - no, sorry. There's a huge missing piece of the puzzle.
Sorry to be so cryptic but it will take me more time than I have to explain. I will come back, but in the meantime, look up Janet Lansbury. Read her articles, listen to her podcasts.