Your find is definitely a 3-groove Minie bullet (a.k.a. "Minie Ball") from the civil war era or slightly after. We civil war relic-diggers commonly call it a "3-ringer" but it actually has grooves in its body, not rings.
Yours is an unfired one which was deliberately flattened by a bored yankee soldier on garrison/occupation duty in St. Augustine. I say it is a yankee bullet because they captured that city in early 1862 without firing a shot, and occupied it for the rest of the war with no Confederate attempt to recapture it. Armed Yankee troops continued to occupy the city through the years of the so-called "Reconstruction" period.
https://en.wikipedia.org/wiki/St._Augustine_in_the_American_Civil_War
I believe your find is a .58-caliber Minie. Being flattened makes your bullet look at lot larger than its original diameter. For certainty, you could weigh it on a Jeweler's Scale, and compare the weight to typical .54 and .58 and .69-caliber Minie bullets. Impact will change a bullet's shape but does not change its weight.
Your bullet was made for use in the US Springfield .58 Rifle, although it could also be fired in an imported British .577 Enfield Rifle. (Contrary to popular belief, Enfields were not imported only by the Confederacy... in the first year of the war the Yankees imported tens-of-thousands of them.)
You said you're a newbie at this, so here's some advice. Where one unfired civil war MILITARY bullet is dug, there are usually more of them nearby, and perhaps other equipment from the soldiers. I suggest you hunt the spot where you dug that bullet very-very closely.
