The most difficult challenge in security is complacency. Not a particular stage of your career, although it normally starts to set in at around 18 months. You start letting your guard down. You get comfortable. Maybe "nothing" ever happens at 10 a.m. in the morning. So you and your buddies meet up for coffee, just kind of kicking back, not really as alert as you should be. Or, you realize that you cannot possibly distrust everyone except your fellow officers. So, you never get suspicious of the lady running the food truck that you get your lunch from every day. So, it totally shocks you when she gets busted as a drug dealer.
Civilians came to my aid more than once. So, don't think they are all "the bad guys." Just stay alert. And, above all else, if a "gut instinct" kicks in and tells you there is something wrong, listen to it. This literally saved my life. The human brain is remarkable. We sometimes try to hard to override what it is trying to tell us.
The beauty of law enforcement is teamwork. You and your fellow officers will watch out for each other in ways no outsider can begin to understand. It's a really good feeling when you are out with a suspect by yourself, and you "know" they will be there within 60 seconds. You can just sort of "Chill," talking to him, not letting on that you know he is wanted on warrants, or whatever, until they get there.
What attracted you to want to go into this field, if I might ask?