Need help in blender

when i connected these 2 vertices by pressing f, on the other side (the mirrored side) it shows a line i cant get rid of, i want the lines to be on the side im working, but not on the side its mirrored

this is really annoying and I’d really appreciate it if you can help

On the top left, click this
image
The select the lines and delete them.

but i want the lines on the side im working, and not on the side its mirrored

I assume that you create that line AFTER you create this face:
image
The line doesn’t have any association with that face so simply delete the face and rebuild it.

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There might be some doubles on your model. Remove them by selecting the entire model (A) and then press W, select the remove double option.