I'm not sure on how much this could work, as I'm more parroting my information/knowledge that I know when it comes to lessening stretch marks and the very few scars I have myself (thankfully none from self harm, I hope you're doing better with that.) This also helps with people who have acne scars.
Exfoliation will definitely be your friend. You can use something like an exfoliation scrub, I personally like to make my own using coconut oil, honey, and granulated sugar. Coconut oil for the moisturizing, honey also helps with moisturizing but it also is an anti-bacterial, and finally the sugar is where the exfoliation comes into play, as it's gentle on the skin and with enough water that sugar will end up dissolving anyway (This is important as the skin on the neck/face especially is very sensitive. It's easy to form microtears in the skin if you use something too abrasive or acidic. Some facial scrubs use bullshit like ground up walnut shells, which obviously do not dissolve in water and WILL cause damage to skin. Just an example.) If you don't want to use a scrub, invest in a pair of exfoliation gloves like these:
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The more important part after exfoliation if you're wanting to lessen your scars, is to
MOISTURIZE. It is extremely important. Skipping this step may as well ensure your scars stick around.
Look for a moisturizer that has Vitamin C AND Vitamin E. Vit. C is important as it helps reduce dark spots, and also in general is an antioxidant that helps to fight free radicals in your body. Free radicals are responsible for skin aging caused by environmental factors, which also in turn fucks around with your scars. Vit. E also helps with free radicals, but it's mainly known for being soothing to the skin and helping keep in moisture, which again, is probably the biggest component for helping reducing scars and/or damage to skin. If the skin is nourished and healthy, then it will in turn continue to stay healthy and work to heal itself.
If you're confused on any of the info or have question, feel free to ask. I don't mind whatsoever.