Please Help My Friends Use My New Name!
Sometimes, a change like this just takes a lot of gentle, but persistent reminders, as when a teenager or adult wants to shed a childhood nickname. It’s not rude to say "It's Kellin now, thanks!" when friends forget. Just keep your tone cheerful and polite.
If Kellin is quite different from your birth name, it may be harder for your friends to shift over to it in their minds. When new names echo the sound of previous names (say, Adam becomes Addy, or Emily becomes Emmett), they seem to be adopted more quickly and smoothly. So if you're asking friends to make a pretty big leap, just know that you may have to hold their hands for a while during the process.
Instead of demanding or complaining (which tend to be ineffective), you might try asking your friends why they find it hard to use your new name. Is it hard to remember for some reason? Do they think it doesn't fit you? Even if the answers to these questions aren't very helpful or revealing, this can be a non-confrontational way to help them focus on using your new name. It will make them more aware that they've been failing to do so, without accusations. The discussion can even reinforce for you, as well as your friends, why you've chosen the name and want to stick up for it. Best of luck, Kellin!