Hi Donny,
Ah, I've been in your position a few times and still competed despite impairments. Like you, I was very prepared and confident prior to sustaining injuries (through overzealous training). Granted, while continuing training and subsequently competing did satisfy the "what if," "you never know," "I would've won" question marks had I not done so, I paid a pretty heavy price afterwards (despite winning in one instance through a lucky quick, one-punch KO and in the other (a wrestling match), losing horribly when, if fully functioning, I could/should have won) which, in retrospect, I'm not sure was worth it. Like Trainer Monkey, I too was overzealous in my younger days, improperly training like a madman, disregarding injuries, and competing when I shouldn't have to my long-term detriment. I'm paying the price for my indiscretions now...

Anyway, what is the extent of your injury at this time? If your shoulder is so bad that it greatly pains you to raise your affected arm, to throw a punch, you're in a bad place and should seriously consider pulling out of the competition. Now, if the pain is relatively "minor" and something you can tolerate, consciously willing to sacrifice some potential long-term consequences in the process, well, you can train around your injured arm, change up your fight plan a bit, and get a cortisone shot before your fights as a temporary fix. I don't recommend you soldiering on, but that's coming from someone who's looking in hindsight. When I was in your shoes, I didn't heed the warnings either; I competed anyway.

Okay, in addition to what I said above, if you are going to compete, I would heed the recommendations you received to drink lots of fluids and take anti-inflammatories such as Advil/Motrin or Alleve. I'm not a proponent of cortisone shots (eschewing them myself when offered), but it's an option to consider. I've had friends say it helped them train and compete pain-free FOR AWHILE. They all ended up paying a price later...

As the saying goes, "pay now or pay later;" when you disregard your body's warning signs that something is wrong, one way or another, you will pay a price. The questions you really need to ask yourself are "How much do I want to pay?" and "Do I want to pay now or pay later?" Is this competition really worth it?

Whatever you decide to do (i.e. pull out of competition, seek medical treatment, rest/rehab, and come back when fully-healed OR soldier on and gut it out with the injury you have), do it with gusto; with full-commitment because anything less will yield unsatisfactory results in you and for you either way.

Take care and good luck with which every way you decide to go.

Best Regards,
Lito