First off, I'm come from a Christian upbringing which I have long since largely abandoned for unimportant reasons. Most of my answers are going to come from my voluntary study of Buddhism and what I believe to be true in my heart of hearts. I do not intend to use this as a platform to preach my sensibilities.
Change. It is a fundamental component of everything which exists. Nothing stays the same. Impermanence is what the buddhists call this concept. It's the idea that nothing can be pinned down to a singular state, or singular idea, or singular concept, etc. Everything is in a constant state of flux, so everything is always simply as it is. This is why labels are considered a major hindrance. We use labels to identify something as it is in a specific point in time, but we also pretend that the thing never changes and is always what we labeled it as, and this causes frustration.
You seem to be focused on this idea that you should be able to control the change that occurs. I think it is very useful to recognize that we as people only have extremely limited control over change, especially change in ourselves.
You clearly have changed yourself, or you would not regret some of the things in your past. Acceptance here would mean recognizing that you are not the same person as the past. The past is the past, it's done and over, and you've changed and learned to regret some of the decisions you've made. The only thing you can do here is forgive yourself for your past. Do not worry about the future or trying to control it, it simply does not exist. The future is a thinly veiled lie we tell ourselves to pretend we are in control of what happens. The only thing you have any hope of controlling is what's happening *right now*. Even then, you shouldn't be focused on controlling outcomes. If you do what you believe to be right in the moment, follow what your heart says, then you will be doing the best you could have done.
Don't focus so much on what you could be controlling or changing, and focus on just living by the principles you believe to be good. Focus on living and enjoying the world unfolding around you *right now*. Because that's all you actually have, there is no past, there is no future, these are both constructs of our mind. If you've made mistakes, forgive yourself, you clearly recognize they are mistakes so you've clearly learned and grown.
In summary, acceptance in this context means recognizing what you can and can not do. What you do and do not control. Loving yourself, including the versions of yourself that have messed up in the past. Those are you too, and those were doing what they thought were right too. If there's something you believe you can fix, fix it. If there's something you cannot control but still care about, then that's all that needs to be done. Awareness and mindfulness of your current situation and the limits of your ability is itself acceptance. Sometimes shitty things will have to be shitty, because there's nothing you can do about that. In these cases know and understand that it's unfavorable, but move on. Don't dwell on it. Focus on what you can do. Focus on the problems you can fix. Focus on the not-shitty things. You might have a massive bill you need to pay but can't afford. That's unfortunate. But you are still alive. You are still able to exist and walk the earth. There are still birds in the trees and squirrels on the ground. You still have your loved ones, and your loved ones still have you. You did things in the past you regret, but you know you can forgive yourself because you did not know you were making a mistake at the time, and you do know that going forwards you will continue doing what you believe to be correct. Just be here, now, with yourself and with the world around you, and do not dwell on things that do not exist or things that you cannot fix.
-charick
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