Best wishes! You're setting out on a unique path. We're more likely than most to understand and support you, no matter what you encounter, but it'll always be your own experience. Try to recall what you learned already from the transplant center. If you feel your, or their, next step(s) are unclear to you, at any time, try to speak directly to your independent donor advocate. You should already have contact information for an individual with that responsibility for you. If you do not, that's your next step, whether or not they think it is. Nothing more should happen until you know the identity of that person, how to reach them, and have spoken to them directly. Among your initial conversational topics, I recommend explaining your desire for explicit confirmation of your, and their next step(s) on an ongoing basis. This isn't unreasonable, and you may have revealed a weakness in their new candidate intake procedures.
That's process. Details vary for each transplant center, so what tests, in what order, when, are all unknowable until they reveal their procedures to you. As I said, considering how far along you already are, someone may have dropped the ball with you. This should have been laid out in writing, and you should already have been asked for your informed consent for testing you've already had, before the tests were performed, including the knowledge of their purpose and where they fit in the context of all you may face. Again, best wishes. Negative crossmatch, type O, and a 4/6 HLA match all are very promising. The blood levels you mention are likely creatinine and BUN, also encouraging.
Your feeling of increasing stress is normal, reasonable, and something to consider. Doing honest self examination of what is causing your stress, whether or not understanding it eases it, will guide you through your own judgement, balancing, and acceptance of the risks you've already starting taking. There are more to come, each one more challenging in its own ways from the earlier, until donation surgery itself. Then they start to tail off, but one of the hardest, and most important to face responsibly, is that they will continue for you from now on, for the rest of your life, whether you donate, or not. This is why I hope you've already got the IDA contact info. If you're deferred after setting out on this path, for whatever reason, you face risks associated with that. If you're not deferred, and become a donor, there's a growing awareness of the lifelong increased personal health management practice we must responsibly undertake.
I've spoken of risk and responsibility. Try not to let that overwhelm you at any time. There's always risk, or, rather, some more or less quantifiable probability of a range of potential outcomes for everything we choose to do, or not do. What of rewards, or the probabilities of positive changes? These are just as important to consider carefully. Take the full measure of enjoyment from the prospect. The reality will be what it is, and you've already changed the odds of what changes lie in store for you and your boyfriend. Bravo!