我从没见过"自制心"这个词，可能这个词根本不存在。

The first form is of classical Chinese style. The second form is of modern Mandarin.

I checked google and it seems that 自製心 is from Japanese since in Japanese, it is common to add "心" to express the meaning of "force/spirit that originated from oneself". About the question you asked, I would like to point out that it is more natural to use "力" (自製力) in Chinese instead of "心". "力" literally means "force" or "power". To answer your question, I think that the phrase is strange like you said. In my opinion, a better to put it would be "而放鬆了自製力". However it maybe wrong depends on the context. Let me know if you have any question. :)

I think the better way could be 而放鬆了自製心. For the words 放鬆, usually there is noun followed or just a short phrase. Another way, if 對 leads a clause. I think it could be 而放鬆了對自製心的控制

i think the it is better to say 而放鬆了自製心，對自製心放鬆了 is not commonly used.