I can't seem to decide on anything! Is something wrong with me mentally?

Maybe or maybe not! Lots of people have trouble making decisions. That does not necessarily mean there is something wrong with you. Unless you have obsessions or compulsions about these decisions. If so, you should talk to your doctor.
Maybe not clinically. But certainly your mental state is telling you something about what you are doing (or not doing) and has you distressed. Are you overly sensitive to making wrong choices? Have you made choices that others criticised you for pretty heavily? Growing up were you subjected to a lot of negative feedback and not much praise? What kinds of things make you indecisive? Do your relationship choices do that?