“Can a stomach ulcer be fixed on its own?”

Hello, first, you want to be sure that it's an ulcer and nothing else, more or less important. It would be best to have a doctor look at you and do some tests to confirm. Normally, ulcers cannot be fixed on their own.

Yes, ulcers can heal on their own sometimes.
Just be careful when saying that you have ulcers. Unless you had an EGD or an Xray to prove the presence of ulcers, it might be something else because many GI symptoms can feel like an ulcer but most of the time an ulcer is not actually present.