Alright, I kinda see where you're getting at. Let me see if I can take a crack at this.
I started developing my universe when I was 12 years old. By then, I had a distinct enough art and storytelling style to catch the attention of some of my buddies. They were so fascinated with my characters that they talked more about them with me than pre-existing fictional characters from other IP's. So it seemed only natural to me that, as I expanded the universe, I'd write in some friend inserts. There are at least four characters in my series that were originally friend inserts. What makes this so funny is that I don't even have a self-insert in my comic. I don't fly that way.
Not one of the friends they were based off of is still my friend today. We essentially had a strong falling out after they found less weird people to hang out with, being that I suffered highly from mental illness and also physical disabilities at the time. I was a weird kid and lots of people started distancing themselves from me or bullying me. So for awhile I had these characters that were supposed to represent my buddies who had effectively left me for people they had better connections with. It happens to everyone at some point or another.
The solution for me was simple. Keep the designs (with minor tweaks) because I worked hard on them and legally owned them, but rewrite the characters to be their own entities. That was a great decision, because my characters are truly mine now and have dynamic personalities and substantially more depth to them. Not to mention, I can make them do whatever the hell I want and nobody can tell me otherwise. One character I changed the name spelling of to signify a coming of good change, from Sage to Saige. Another character I actually withdrew from the main party and wrote into a background character. She now works at a medieval Starbucks because I find it humerous. I felt obligated originally to write her into the party because it would be more fair to my IRL friend, even if it clustered the group and made a whole lot less sense. My friend also shipped her with my male main protagonist which was....a disaster. So I eliminated that in the process.
Now that I've revamped the characters, the story flows so much better, characters aren't hollow shells, I have better creative freedoms, and I don't have to worry about thinking of my lost friends everytime I draw these characters. I think that if you step back and reevaluate your characters and make changes where necessary, it could really help them feel like your own.
Take this opportunity to reestablish the characters how you would've done all along. Give them strengths and weaknesses that you see fit. Make them yours. Make it your story. Redesign their outfits or hairdos or eye color or whatever you want, how you personally thinks best. The co-creators already gave up on you, so show them why and how you were better independently to begin with. Really make those characters shine, my friend.