I've been doing attachment-based EMDR for the better part of two years.
Sad Elf more or less describes the process of EMDR itself, your therapist will ask you to focus on a memory, then turn on bilateral stimulation (vibrating cubes you hold in your hand called "tappers," because you can't call them "vibrators"; headphones; or a light bar. "Bilateral stimulation" just means it alternates left-right-left and so on). Periodically they will stop and ask you, "What do you notice?" You tell them and then the process repeats itself.
There are different stages of EMDR; for the first stage you will do history taking and then something called "resource building" where your therapist makes sure you can regulate when difficult stuff comes up. As Sad Elf noted your mind may not "want" you to bring stuff to the surface or you may not be ready. The whole point of therapy is to make seen the unseen so you can process things you haven't processed before, that are causing your symptoms. It sucks and it's awful but once you process them then you can be present and unaffected by the past. It's really, really important to note that therapy is supposed to be uncomfortable. You are letting out all the suppressed feelings that are giving you your symptoms. That process is supposed to be hard. But the result is that you don't have those feelings suppressed anymore and you can life your life in the present.
So yes, it will absolutely get worse. But then, it will get better. Think of it like a broken bone that didn't heal correctly. You might have to re-break the bone so it can heal correctly. Yes, for a while, that will hurt worse than having it healed incorrectly. But wouldn't you rather have a bone that's correctly healed? Even if it hurts again for a while on the way there?
Depending on how much trauma you have this might take some time because if you've had multiple traumas (A) your mind won't want to let it out all at once and (B) you can't process all that at once. Plus you're probably really good at avoiding it. So that's why I've been at it for almost two years. But it has been worth it so far, a lot about me has changed. I'm better at noticing unhelpful patterns and setting boundaries and have different beliefs about myself. A good therapist will let you lead and not push you or make you talk about things until you're ready.
I called it attachment-based EMDR because while the relationship you have with your therapist doesn't matter as much as with, say, person-centered therapy, it does still matter. Even in EMDR, the relationship you have with your therapist is, statistically speaking, the most important indicator of success in therapy, that you and your therapist have control over (more even than the method of therapy!).
Hope that helps.
