To touch on two things that unworthy and w,h mention...
Yes, remember that it is not so much the "acting" like a teenager, but they THINK like a teenager. There is nothing wrong with going out, having fun, even "letting it all out", now and then...
What is telling is the rationalization, justification, and irresponsible or unresponsible thoughts that go on in the minds of a MLCer. Looking only at the pleasure of a moment, without considering the harm it may cause anyone in the moment or in the future.
My W does a lot of things still, and thinks the kids are OK and will be OK, being witness to these pursuits for her happiness. They aren't OK, directly and indirectly due to W's and my behaviours and what has and is going on between the two of us. But she can't / won't see it or acknowledge it.
On the gift thing, while it may not be true that OW was intentionally trying to hurt you or push your buttons, considering that as a possibility might help you work through what is going on. Same goes for your H. You can assume love, from your H but you can also be open to possible motivations other than a misguided attempt to love you that isn't working.
One of W's OM has been in the habit of engaging my kids in a way that, generally speaking, only really good friends or relatives (or parents) might engage kids. Including that of buying gifts. While I strongly suspect that he is trying to by my W's love, by catering to the kids directly in this way, I could also be open to the idea that it is some unconscious dig at me by him.
No matter how offensive it might feel for me, if he wants my W, he can have her, but they are my kids and no matter what he does, that can never change.
