Congratulations on your marriage op!
My advice would be to chill. This is new to you and it’s completely normal to feel a bit nervous and worry and not enjoy it as much as when you are at ease and know what you are doing. What’s the rush? You and your wife are just at the start of a new adventure together. Take your time. And talk to her and take her along with you, The responsibility of your sex life is not all on you; she will have ideas, preferences, insecurities and worries too, So make sure you don’t forget to tell her how attractive you find her,
The past is the past and it’s pointless thinking about decisions back then, so try and stay in the present.
Being a woman, I can’t advise about mechanics specifically but my guess is that you are thinking far too much about staying hard, you are living in your head too much, and putting far too much pressure on yourself.
I understand that you genuinely are feeling this when you say,
“It really effects and the failure sits deep within my core right now”
But I would really like to challenge that viewpoint of yourself.
You are definitely not a failure! The first couple of times having penetrative sex with someone else is usually pretty bad and that’s completely normal. Sex is like cooking, the more you do, the better you will get at it!
And once you become proficient in the basic techniques, like putting a pillow under your wife’s hips maybe to change the angle, then you can improvise more confidently and be more spontaneous.
Relax! Give yourself and your wife time to explore one another properly without any thought of piv to begin with.
Take the pressure off yourself. Give your wife a massage with scented oils. Or sit in the bath together, with you behind her, and drink some wine. Talk to each other. She will love it! Maybe stop masturbating for now, except mutually, and take the pressure off penetrative sex being the be-all-and-end-all, because the very good news for you is that for most women, it really isn’t!
Honestly, this is you putting pressure on you, and as a long-married woman I can categorically say that it’s nice to have a hard penis some of the time but piv sex is absolutely not where most average women find the most pleasure, especially in the missionary position. Our pleasure is much more centred around the clitoris and what a man does with his hands and mouth is equally important, if not more important, than penetration.
Also men are just human. My dh is older now and his erection fluctuates according to tiredness, stress. Most women totally understand this, His advice I know for great sex would be, focus on your partner and their pleasure and not so much on your own performance. Make sure she has an orgasm before you penetrate her if at all possible and be prepared to switch things up a bit to take the pressure off you, Try different positions, toys, and most of all, relax. You are focusing too much on the end destination while not enjoying the journey,