I've actually had this test TWICE. I can tell you about it. It's not comfortable, but it's not the worst thing ever. It is a pretty quick test, just a couple of mins. I have interstitial cystitis (where you get a lot of UTI's and UTI symptoms without any actual infection because the bladder is irritated) and have had my bladder checked twice in the past like ten years.
the actual test is they lube the camera thing and put it into the urethra which feels pretty weird and uncomfortable. Sometimes they might fill your bladder with liquid to see the bladder capacity (this part sucks because it feels like you have to pee like crazy) and then they drain out the water (yay relief and you don't even have to go to the toilet). They check the bladder lining and everything and it's very quick.
After the test your urethra may feel a little sore and when you pee it may sting REALLY bad. Now the first time I had the test done it was really sharp like a knife when I peed for the next day and I had a wee tiny tinge of blood (the urethra is extremely delicate tissue and is irritated easily). That was really the worst part of it, the peeing afterward for the day. The second test I had I actually didn't have very much pain when I peed. It was much easier and no blood at all, so I guess it depends on if your tissue is already irritated or the skill of the doctor or if there's enough lube, I don't know.
(I've had a catheter before as well and has no issues with pain peeing after so I think it's irritation from the scope they use.)
The doctor WILL give you an antibiotic to take to head off a UTI. Usually it's one pill you take right away when you get home. You can also take something called D-Mannose which is a sugar (find in a health food store) that helps to prevent bacteria from sticking to your bladder wall. If you take some D Mannose the day of your test and also after, it can help flush out bacteria. I also found that D Mannose helped my symptoms when I had a UTI or UTI symptoms. It's also great if you get a lot of UTI's after sex, you just take some an hour or so before sex and a day or so after. It's a sugar so you dissolve it in a drink and drink it (I put it in apple juice though I think it works better if not in an acidic liquid). Also drink cranberry juice which makes the bladder inhospitable to bacteria.
You're going to be fine. I know the test sounds scary and icky and it's uncomfortable but it's quick and I think the worst of it is just having anything touching the urethra since it's such a sensitive part. (I don't know if men understand it as well since their urethras usually keep bacteria out. Women have short urethras so we get a lot of irritation and pain and stuff there, even from SOAPS!)
Just be aware of the sharp sting when you pee for the first time after. It may happen, it may not, but that's what I actually thought was the worst part besides when they filled the bladder (apparently I have a small bladder capacity, yours may be better and you may not feel what I felt.)
If you can take a tranquilizer that could help a lot, but also tell yourself this, you're going to be fine. It is quick, and you're going to get these issues taken care of. Bladder/Urethra issues suck. Also make sure your urologist talks about pelvic floor stuff because I have been told that some of my issues are due to pelvic floor muscles and I need pelvic floor muscle therapy (I couldn't afford it so I never got it). Those muscles and doing Kegels and stuff can really help your bladder so ask the doctor about that as well.