After a few months of depression, research, arguments, talking to people on here, and endless ruminating over what is wrong with me... I think I've found it.
Frankly I can't believe the only personality disorder I've been diagnosed with is OCPD. Is it common to have more than one PD? Seems to be pretty common.

I think I have OCPD, APD and DPD. I'm just about ready to see a clinical psych, should get confirmation there. But what I'm reading about these disorders makes sense. It totally resonates with me.
The thing is, some other diagnoses have too. But... they are diagnoses that can explain my symptoms, the same ones I think are explained by the PDs. Since PDs are kinda all-encompassing, is it surprising that I've found lots of other conditions that almost explain all my problems, but not quite? I know I have low self-esteem, and that's part of a lot of conditions.

But I'm pretty sure this is it. Avoidant I identify with most, followed by Dependent. I imagine many people suffer from both.

Yep. I absolutely hate customer service. I like my dry cleaning job because most of my work is done alone, with clothes instead of people. When customers come in, I'm intensely uncomfortable. The tiniest things scare me. I worry about asking them how they're doing, because I don't think I'll sound sincere. Then if they ask me first, I feel like they must think I'm a terrible person for not asking them. When I'm waiting for their receipt to print, I worry that maybe I'm supposed to say something to fill the silence. The whole time I'm sure I'm going to do or say something that will make them think I'm totally weird. As soon as they leave I know I'm being stupid, I know for my part I barely pay attention to cashiers, and certainly don't care if they really care how my day was. I know it doesn't matter, won't get me fired, probably won't even result in a personal or formal complaint. And I'm "used to" it, as in I know it's going to happen every time. Knowing it helps me to not freak out as much, but it doesn't stop it.

I generally hang back until someone expresses interest in me. Then I feel a little more comfortable, but I do find myself worrying that I can't tell the difference between politeness and true interest.

This one to a lesser degree... the people I'm close with, I'm much more comfortable with. If they've been around this long, they must not think I'm a freak. But I definitely avoid saying things I think will make my boyfriend mad, and when I make decisions often have to run through a check-list in my head, to see if there's any reason it could make him mad.
Generally I'm really open with people I'm close with, but sometimes when I'm feeling particularly bad I feel like it's impossible to explain, I'll open my mouth and literally feel like the words get stuck, and I tell someone I'm fine even though I know they can tell I'm not, and I can't even make my voice convincing.

I think all the other stuff covers this.

I freak out when I have a new "social responsibility", like making phone calls or asking customers if they want to donate to a charity. I definitely do feel like people won't like me unless I give them some reason to.

Hard not to. A five-year-old could probably be a better customer service rep than me.

Absolutely. I've never sang karaoke, even though I kind of want to. I've seriously thought about practicing a song ahead of time so I sound good, and I know how ridiculous that is, because karaoke is RARELY good. I hated gym class in school and never wanted to play any new games. Once I also had a breakdown when my father asked me to play pool in front of other adults at a party. I knew I wouldn't be able to hit the ball, and couldn't bring myself to try. His insistence only made me more scared, and he finally gave up and seemed disgusted with me. I was very close with my father, and this memory stands out as one of the times I felt like I failed him. I was trying to avoid looking stupid, but in being so reluctant I feel like I embarrassed him, which is worse.

That's 7/7. Think it's safe to say it's a PD?

There's a few other things I'm curious about. How often are Avoidant and Dependent PDs diagnosed together? They seem very similar to me, and the only symptom of Dependent that I don't see as fulfilling both disorders is the unrealistic preoccupation with fears of being left alone. If you asked me if I could be alone, I'd say yes. If you asked me if I can take care of myself, I'd say yes. But the few times I've had a real reason to think my boyfriend might leave me, I felt like the world was ending. The thought that I couldn't live without him kept popping into my head, even though I couldn't explain why, or even what that meant.
But it's likely I have this one too. I do have to ask advice a lot, to the point it bothers my boyfriend. He's always saying I need to figure stuff out for myself, but it's been proven again and again that I make bad decisions, so I'm afraid to trust my own judgment! For a long time I depended on him to drive me places because I'm terrified of driving. I was finally becoming less uncomfortable with it when we totaled our car, so that's a moot point now. But he always takes care of our money pretty much. At least, he's the one who knows the budget, and it doesn't seem like he really wants me to take over on it, though I know it stresses him out to deal with it. I actually "pay" the rent, as in write out the check and take it to the office, but it feels like a menial task, and like he's still ultimately in control.
I feel like we have this weird dynamic of him taking my responsibilities, but then telling me I need to fend for myself. It gets even harder for me, because we're in the pattern of him taking care of that stuff. Plus, like I said, I don't trust my judgment, and it seems like he doesn't either.

Anyway this is getting long-winded, so just one more question: what type of therapy do you find most helpful? I was thinking CBT or DBT might be good for me. Is that still the best option for PDs, since they creep into every aspect of your life and all you can do is combat specific behaviors?

Also, sort of a rhetorical question... but how come I've been diagnosed with OCPD when it's the one of the Cluster C PDs that I fulfill the least criteria of? Then again I was diagnosed when I was 17, and I suppose a lot of symptoms of Avoidant and Dependent could just be seen as being a teenager. Especially Dependent.