You responded very well under the circumstances and yes, it does take experience to reduce the amount of times one looks back and says to themselves, "I should've".
Again, you expressed non-consent to the search and by expressing that you will not obstruct or by remaining in the area does not invalidate your non-consent. The agents acted under "color of law" with coercive authority while armed. You invoked the protections under the 4th Amendment to which they have no leg to stand on period.
Here is an excerpt from a class/lecture given at the Federal Law Enforcement Training Center regarding the 4th Amendment and consent;
Solari: ... Lawful consent has to meet three requirements. First, it has to be voluntarily - a product of free will. Second, the consent has to be given by someone with either actual or apparent authority over that place to be searched. Third the officers must confine their search to the place authorized.
Miller: How do courts determine whether consents is voluntary?
Solari: By the totality of the circumstances. The courts appear to find consent voluntary so long as it’s not the product of expressed or implied coercion. For instance, the courts consider the suspect’s age, intelligence, the number of officers who are present, the tone of voice they used, whether the suspect was in custody and given Miranda rights, and the suspect’s experience in the criminal justice system. No single one of those factors is determinative. The courts are going to consider all of them. Officers don’t have to tell people that they have the right to refuse a search, but of course if they do, that’s going to weigh heavily in favor of finding that the consent is voluntarily.
Miller: Well let’s get back to our resident at 123 Main Street. Let’s assume these facts. We’ve identified the occupant as the owner of the car in the parking lot. I ask the occupant, “Can we search your car?” I explain, “You have a right to refuse; however, if you don’t let us search the car, I’m going to apply for a warrant.” He then said “okay.” I spoke in a conversation tone of voice; however, this guy is in handcuffs at the time I’m having this conversation with him. Do you think that would be a voluntary consent?
Solari: I think it would be. What would weigh against a determination that the consent is voluntary is the fact that you’ve taken him into custody, he’s under arrest, and in handcuffs at the time you’re having that conversation; but; that doesn’t necessarily mean you can’t obtain a voluntary consent to search the car.
The things that definitely weigh in favor of it being voluntary – first, you told him where you wanted to search. You actually told him, even though you didn’t have to, that he had a right to refuse to give you consent to search. You also told him that if you don’t let us search the car, “I’m going to apply for a warrant.” Now that verbiage is pretty important. You simply told him the facts - you’re going to go apply for a warrant if he failed to consent. Importantly, what you can’t do is to tell him that if you don’t consent, I’m going to get a warrant or somehow pretend that you have a warrant that you don’t already have. But all you told him was that you were going to go ask for one. So I think that would be voluntary consent.
... ... ...
Miller: After the consenter authorizes the search, can he withdraw?
Solari: Absolutely. You bet. However, the courts generally require that withdraw of consent be unequivocal. By that, I mean, the consenter has to then make it perfectly clear to the officer that he no longer consents to the search and wants the officer to stop.
http://www.fletc.gov/training/progr...amendment-transcripts/consent-to-search.html/
