It's ok to think that, don't forget it's all part of the process. But the best advice I can give you is to start thinking about the future and not the past. You've done things in the past that you're not proud of, but you're changing that habit, so focus on your breathing and pay attention to your body.
This question and answer thing is so random, I’m not sure how it chooses who to send the questions to. But I do have an irregular meditation practice and had a regular one when I was much younger.. You asked how to not let those things that your mind comes back to bother you… I believe for me the answer is in acknowledging and accepting the feelings that come up.. See them, be present with them, and try not to judge yourself for feeling that way. Perhaps a focus on self-compassion? I’m sure there is literature on this subject, particularly from the Buddhist perspective. Once a feeling has been fully acknowledged, it ceases to have the same power. Because you begin to know yourself as the observer of the feelings. Your feelings are happening to you but they are not you, you are so much more!