As is typically the case. monkeyflower is right on target. I will deal with the easy stuff, your risk of HIV or other STD first, then turn to the more important topic, your mental health.
There is no risk from having a partner's spit applied to your penis. No risk of STD or HIV of any sort. Before you go there, this is true wether or not your partner had a sore in her mouth or whether opr not there was blood in her saliva. You need not be concerned, nor do you need further testing. And yes, you can have sex with your girlfriend.
Now let's deal with your fears and anxiety. The fact that you have sen many of our answers to similar questions yet express such fear and anxiety concerns me. I would urge you to either talk about this with your own personal health care provider or, should he/she minimize your concerns or not fully address them, seek the assistance of a trained mental health provider.
I hope this helps. EWH
I don't mean to jump in before either of the doctors reply, but your last paragraph concerned me. Those kinds of thoughts can be serious. If you haven't already done so, please talk to your family doctor about how you're feeling. I also urge you to consider counseling. In the meantime, if you're in the US and having thoughts of suicide, please call 1-800-suicide, or call 911.