Truth is often stranger than fiction.
I have received countless promising submissions that I end up turning down for this reason; the character encountered a situation, and dealt with it in an unbelievable manner.
When questioning the writer about this part of their story they often tell me, “But it really happened like that!” I have no reason to doubt them. My own personal story holds many such incredible circumstances, but they would not come across as believable by any stretch of the imagination to a reader of fiction.
A writer must be willing to edit out the unbelievable. And for some, that feels like they are being untrue to themselves. But if their personal life experience brought them to a good story, it has served a grand purpose and the writer needs to be content with this.
Remember: ALWAYS keep your reader in mind. And be willing to cut those unbelievable segments of your story out.
Here is a list of articles a Google search supplied on writing believable characters.
Read through a few when you have the time. There will certainly be a nugget or two here that might, when applied, strengthen and enrich your story and characters.
And be encouraged, Spring is around the corner!