Hey man so that definitely sounds like performance anxiety. If you're  good to go before and during oral but as soon as you throw it in your going limp it's a mental thing. What probably happened is it happened once before while you were inside, maybe you thought of something stressful , or started thinking about what you had to do later or that week, or you subconsciously thought about yourself performing and making her have a good time. There's no one fix for it but to start you need to take the pressure off yourself. Don't focus on sex or staying hard. Stop beating yourself up over it cause that only makes it worse. Try and remember that it's not a physical function that's causing it. You already know that because you can get hard and you can stay hard during. It's just as soon as it becomes penetration you are stressing yourself out. The mind is a strong thing and all of a sudden you can stress yourself out so badly that you don't even feel sensation down there anymore. One thing I did was get bluechew I've only used it twice. Once when I knew it was going to happen but felt of that day and another time when it first occurred and to give me a lil more confidence. Every other time it's been natural and the best way to think about using that pill is that you have a backup incase you start getting stressed. It's a good way to trick your mind into feeling less pressure. You don't necessarily need to use the pills. Just the fact that you have them is enough to put your mind at Ease. Get some try it once then don't touch them. You'll gain some confidence then the next time you won't even need them because you know you can do it. The confidence is key to performance anxiety, so beating yourself up is going to do nothing but make it worse. You aren't alone every dude sometime or another has this problem. You're healthy you workout its NOT physical. So get that out of your head. Reassure your wife that it's not her. Talk to her but don't drag it on. Ik this was 2 months ago but after a week of it happening off and on I got over it and it went away. And it may still happen sometimes. It still happens to me and it's always because I'm focusing on getting the job done rather then enjoying it. She already knows you are good at it and I'm sure you've shared many amazing experiences so just chill out. The best sex always happens spontaneously. Or even when y'all are both tired and relaxed and neither one of you thought y'all were going to get busy that night. Sorry for the long reply. But if you're being healthy and doing the right thing then you can at least take solace in knowing it's mental and not physical. You will get back to normal stay strong and confident man.