letting go...

I'm taking a mindfulness course designed to help me to recognize my emotions and the causes for them.  In theory, by doing this, I'll be more in tune with my body and be able to stop a depression decline before it's too late, without medication.  I love theories.
Lately I've realized that I am recognizing my body's reaction to certain situations, namely the incredible tensing that occurs with stress or unpleasant encounters.  I've got that portion down pat.  The next step is more difficult.  I'm supposed to acknowledge my reaction and emotion and let it go or rather let it be.  This sounds way more simple than it actually is.
There are times I succeed, and perhaps after years of practice I will be able to do this without a struggle.  Then there are the times, when a part of me doesn't want to let it go or let it be.  Is it wrong to want to hold onto to and savor the anger and frustration a little?  It is so much easier to feel dread and distaste when I've already prepared my body and mind for them.  If I truly let it go, I am opening myself up to, admittedly, the possibility that my next encounter will be positive and devoid of these feelings, but I am also leaving myself vulnerable to the next attack.  There are certain people in my life who rarely let me down in this regard.  Every encounter leaves me tense and angry, sometimes sobbing.  Can I acknowledge, forgive and let go even though the next encounter could be bad enough to open the rabbit hole's door?
On the other hand, if I don't, am I setting us up for continued strained relations?