For me I have four things consistently helpful. First, exercise. Moving around a bunch makes me feel better, especially if there's a feeling of accomplishment attached. For example, I've felt really down a couple times this semester and going to a martial arts club practice has consistently lifted me out of it. I went from "I don't belong here; I'm a terrible student" to "I did well that practice!" There is a trap here if you go in with too high expectations, though.

Second, actually accomplish something, preferably something that has a tangible benefit. If you can physically build something, that would be the best. But even silly things can work. I've felt out of place at law school, but I use my LSAT score to remind myself that I'm skilled enough to be here. By "actually accomplish" I'm trying to get at something you can't tear down with low self-esteem. No matter how bad I feel about myself, I can't reasonably deny that my LSAT score is good. I think physical creations are even better because it is a physical not just mental reminder that you did something good.

Third, find interruptions for those trains of thought. For me, I find that being in the presence of friends makes it hard for me to dwell on my negative self-image.

Finally, create positive comment trades with friends and family, if possible. I know a lot of guys do the insult to become friends thing. Personally, it's never worked for me. When I do tease people, it's always within a context where I'm already saying positive things about them. For the most part, my close friends do the same. I think the comments people make about us mean more to us than we think. Why have friends reiterate the painful internal monologue we already have? Instead, specific and true positive comments can disrupt that narrative.

For example, one of my closest friends is a talented leader. She is great at organizing and bringing people together. Thanks to bad experiences, she has a negative internal narrative where she isn't good at it. A leadership opportunity came up and she planned on passing on it because of her belief. Instead, I and another of our friends were able to point out specific instances in which she did well and drew parallels between the things she had done well and the new opportunity. By interrupting and contradicting those negative internal messages, she first became more open to considering the position, then actually went for it.

It's great you are working to make your life better! Low self-esteem/self-image hurts, but you can make it better. One last thing, don't make feeling better contingent on future action, even the things I listed. "I'll feel good about myself if I exercise today" is dangerous because if something happens and you don't do it, you'll reinforce the negative narrative. Turn the past into precedent and a motivation for today. "I exercised yesterday and felt great; I want to do that again today." When possible, frame your actions positively. A positive internal monologue motivates action more than a negative one.