I do not panic over texts, but do constantly fret over whether or not I am a burden on people, or if I have done something to make my partner mad, etc etc.
I think it comes from the same place... I assume your PTSD stems from abuse? Mine does.
I am pretty sure it is a self worth thing. Our abusers did their best to convince us that we are somehow inherantly wrong, broken, awful, and otherwise worthless. That we just exist to fail and mess things up... or we convince ourselves that we do while struggling to keep it together dealing with all our symptoms.
I would suggest talking with your friends, or instead of apologizing for long texts directly, just thank them for listening. You do not need to apologize for being a person.
And with texts... don't forget how many things can distract someone from responding back. Work, family, errands, another call, or just plain forgetfullness. If the person you are texting is someone who cares about you they will get back when they can.