Let's look at some good stuff you admitted here: you were talkative for an hour or two, even though you felt tense. And even though you felt tense, you were able to also slow down and take it easy. -That's an achievement, and you deserve to give yourself credit for that. Before therapy, could you have imagined talking to someone more calmly for an hour or two? Most of us could not.
Setbacks are a sign of progress.
It's also good to know your limits. You do have a right to also set limits on what you accept, on your time. I know this is hard - it's hard to say "no" to people, and friends who invite you out. But, remember, try to rationally tell yourself that you also have a right to say, "I really enjoyed meeting with you. I need to go now, feeling pretty tired." Perhaps you can say yes and be proactive in these situations, but also be proactive about believing in yourself, that you have the right to set a time and to be okay with that. "Normal" people will excuse themselves all the time. "Sorry, I have to go now, ...pretty late and I have an early morning... I have some work to finish up, etc..."
Maybe knowing your limits to certain situations will also give you some calm, that you are not facing a situation with no end in sight. And each time you face this situation, you can gauge yourself whether or not you can extend the time, stretch beyond or respond to how you feel.
Keep yourself healthy too, rest and exercise. This will increase the chance of more "good days".
Remember the Deserving Statements. You have the right.... You know when I ask my friends out, they don't always say yes. Does that bother me? No. They have lives, too. We do need to practice being proactive, and remember not to go with our emotions. We need to react on the rational. I would argue that your feelings of exhaustion, though they might seem to come from nowhere on bad days, they probably come from ANTs thinking/feeling, subtle ones that you may not be catching. The exhaustion we feel is real, but it is also an ANT, and sometimes when we act against that feeling of exhaustion and DO something, we feel more energized.
But, you are doing things and saying yes - so I congratulate you on a job well done. Know yourself and your limits too. Try not to get angry at yourself for not succeeding EVERY time. That is impossible. We don't need such expectations.
People get their energy back in a few ways - I think we've all heard of this and know this. Some from being around people, they thrive off social energy and that makes them feel more energized. Some get energy back from time alone, even though this person might also love socializing - time alone is still what is needed. And I don't think it's true that all socially anxious people are introverts who get energy from time alone. Some are extroverts who got trapped in social anxiety just like introverts who get trapped in social anxiety. Either way, be honest with yourself and do the right thing to get your energy back when you are actually alone. Do the things which indeed do make you better able to come back to a social situation more ready and with more of an ability to do well. Try to make sure your habits during your alone time are helping restore that energy and not unhealthy habits that don't actually give you energy back before going back "out there" again. What are those habits? - Well that is for you to know. For me, I am better if I unplug from computers and TV. It's better if I read a book, get outside for a walk, do some exercise. I could simply surf the internet in my free time or watch movies, which is passively fun and also good sometimes, but I know those activities don't really restore my energy. They are just easy habits to fall into.