I'm not sure if I can add anything to what the others have said other than to add my vote of confidence to you & say a massive congratulations!! You've done so well to lose the weight that you have & on that fact alone you should be proud of yourself & what you have done so far.
Yes you still have more you want to achieve, but losing weight & gaining fitness all takes time - not one single person you see at a triathlon or at the gym suddenly woke up one morning looking like & able to achieve what they do, they too all had to start somewhere the same as you.
It's fantastic that your boyfriend will be there to cheer you on, and if for some reason you come last then who gives a toss - the fact that you are there & you are doing it is the most important thing!! There are millions of other people who will be sitting on their couches doing nothing, but not you, you are out there trying and giving it your best and that is all that matters at the end of the day!!
No-one will be looking at you going "what is she doing here" - everyone will just be saying well done and congratulations for giving it go, and for everything that you have done you should also be patting yourself on the back instead of berating yourself!
Please stop stressing about it - be proud of what you are doing and what you have achieved so far and just soak up the atmosphere and enjoy the day (as tri's are one of the most friendly sports around, honestly), but most of all have fun, as that is what it is all about!
Yes it might all seem hard now, but as I said we all have to start somewhere and this is the start of something wonderful & fantastic for you - I can guarantee you that if you keep at it and you book to do the same event next year then you will smash your time, but this is your starting point and the only place you can go from here is up!
Good luck & we expect a report afterwards!
(and by the way, this is coming from someone who wears glasses (although I am lucky & get on with contacts so that is my way around it, but as others have said ask a marshall if you can leave them somewhere close to the water & safe) and has also lost nearly 7 stones in weight, so you definitely are not alone)