As always, when there is a sudden change in your body, it doesn't hurt to consult a doc to make sure nothing more serious is happening. It also makes sense to run a quick review through some of the usual suspects: stress, diet, medication. As we get older, it takes less and less to cause a disturbance in the expected function of the body...
From your post, it doesn't seem that medication changes are a candidate... so that leaves stress and diet from the list... With xmas around the corner, and absent much more detail, I would bet my first token on stress being the culprit. Planning the time with family, dealing with the rat race / gift buying, likely dreading some of the people you're going to have to spend time with... they all add up, and likely compound.
On top of that, it happened once.... it freaked you out a little as it's not normal for you. The next time, you were a little anxious about it, more stress, and it happened again. It's very easy to fall into one of these negative feedback loops.
Has your load changed at work? How well rested are you? Are you meditating, or at least napping, to recharge during the day? Has your diet changed at all in a significant way? Are you getting enough Vitamin D (aka sunlight)?
It may also be worthwhile to investigate this "takes a while to get ready".... it kinda sounds like you're doing that by yourself... Some foreplay is essential to warm up the body... and it's very natural to have a weak erection absent it.
Further, the style of sex you describe puts a lot of pressure on your erection... Which in itself isn't bad, different strokes for different folks, but leaves you guys hanging if the erection decided not to show up... which is only going to reinforce that negative loop you seem to be caught in.
I would advise adding some different forms of having sex to your repertoire to at the very least break the cycle. Have some sessions together where you just kiss and use hands on each other. Have some more that take it to oral. Have some just oral sessions... Take turns on each other, and take turns pleasing the other without being pleased back.
Posted: 15 Dec 20:41