What makes you believe this would be effective in the first place? Is there any research supporting this method of self improvement? I cannot see how spamming yourself with emails with vague positive thinking messages would help anything.
I am not sure what specific change you are trying to achieve from your question. Are you? It seems you're looking to be just "better". With a goal that vague and undefined, it's no wonder you're having trouble.
I'm not trying to be down on you for wanting to improve yourself. I'm trying to highlight that you need to specifically identify what you want to change. Perhaps there's a lot of things, and that's okay, but make a list of specific aspects of your life or yourself. Identify concrete, actionable steps you can take that will help you get closer to that ideal or achieve that task. For example you listed "Stay positive". Okay, great, now how do you achieve that? You might identify situations in which you get down on yourself, you give up hope, or you get angry. Make a list of them, and focus on changing the way that you think about the situation when it comes up, or the way you choose to react to it, or the way you talk to yourself in your own head when it happens. Or, spend a few minutes every night writing about how you handled a situation that's bothering you, or one that you handled well. That's just an example, you have to do this for yourself to cater to your own situation.
I guess what I'm getting at is that I don't think you will change your life with directionless reminders. Reminders to take specific, directed, intentional action, however, may be much more useful. Also, beware of splitting your focus up across too many areas.