Hello and welcome to BW forums

Oh dear, you sound like you're at your wits end with those naps.
The good news is that it's likely he just needs an A time increase, his resistance to naps (yes he does know a nap is on the cards and he's trying to tell you he isn't quite ready for one yet) and his 45 min naps are both signs that he is UT (under tired).
The other good news is that as he was previously an independent sleeper and self settled it likely won't take very much to get that back.
The other (possibly less good) news is that this could be the 4 month regression come a bit early (mine was 3.5 months) which is a prime time for LOs naps to go off track for a while, you are not alone in this being a difficult phase. But they do come out the other side!
As you seem to already know your routine you might be able to work this out yourself, here's the guidance A time for age, but it is just guidance, if he's on too low an A time you can start right away to increase (15 mins, hold there a few days then increase again if needed), if he's already at the upper end of A times he might like long A times (mine did, especially the first A time of the day).
Average A times- BOOKMARK ME! If you are unsure you can post your EASY times (the real times of what happens in a day rather than what you hope for) and we can have a look to see what might be changed.
If he's never been swaddled, never had a dummy and never had a long wind down I wouldn't introduce any of those now. Sounds like he is great at self settling but just needs to get his routine back on track.
He is probably also becoming more aware of his surroundings and sleepy signs might not be as reliable as they were before. He may need a different activity rather than being put to nap. You can move to a different room or show him a different toy/object for a few more minutes to help fill the additional A time.
As you are so tired, it might also be time to call in back up if you have any on offer. Is there a friend or relative who could mind him during the A time so you can rest then, and you can take over when the harder part of putting him down to nap comes?
Stay strong, you didn't do anything wrong, it gets hard for all of us at one time or another.