Last I checked you had to be 21 to purchase a hand gun. As much as I loved to I don't think I can.
Purchase, yes. Possess, no. You can be given a handgun as a gift, and own, possess, and carry it (OC) legally in Nebraska.
I note that I tend to agree with most people here: your choices have given you significant amounts of information about various threats in your environment. Carrying a shotgun (either in hand or in a sling in combat mode, because having it slung on your back won't help you at all in a surprise situation, which it what has apparently happened to you before) will 1) escalate the situation assuming one happens (after all, you have apparently not been physically damaged from your muggings), 2) potentially provide a free gun to a mugger if your awareness level does not suffice for the situation, 3) cause a significant amount of legal issues for you if you do not make correct decisions regarding use of force regarding lethal force in response to a mugging if your awareness is enough to not provide a free gun to a mugger, and 4) garner a significant amount of negative attention from the people around you, in particular the business where you are going.
This isn't me telling you what to do---I'm merely saying that carrying a shotgun turns your low-level (not saying "not important," merely saying "no physical damage") interactions with criminals into one significantly more dangerous. Particularly as you are planning on carrying a large, obvious weapon in an area apparently filled with people who already carry weapons and have no problem taking things from other people---where you will have difficulty walking around with a shotgun at the ready the entire time.
I'm all for people defending themselves. That being said, effective self-defense (that keeps you safe which includes both keeping you out of jail AND keeping you from paying large legal fees) in this case probably should really be more based on making intelligent choices (example: not going for smokes when you know people like to mug you----and as someone who worked the night shift for two years in grad school, we both know full well that you could think ahead and get smokes at other times), and being aware of your surroundings and the people in them.
Considering the "at gun point" and "at knife point" parts of what you what, I'm curious as to what the police did when you reported the muggings. I assume you gave them a description of your assailant in both cases?