Peculiar pagebreak problem with nested environments

Peculiar pagebreak problem with nested environments

I have been trying to make custom theorem environments because the
classical newtheorem command does not suit my needs. I have been stuck
with a pagebreak problem which I explain below. Here is the code :
\renewenvironment{proof}
{
\textit{\underline{Proof.}}
\vspace{12 pt}
\begin{proofline}
}
{
\end{proofline}
}
proofline is an environment which allows me to do what I want inside my
proof environment (more precisely, puts a long vertical line at the left
of my text). It ''surrounds'' the text very nicely and makes it quick to
skip a proof if desired, one just looks where the line ends (or
alternatively where the square is). The actual environment is empty but it
has \surroundwithmdframed on it.
Anyway. So what I want is my ''Proof.'' part to stick with the beginning
of the proof. I usually encounter the following output :
Proof.
<----Page break---->
Beginning of the proof...
Sometimes adding \* after the Proof. line works (by sometimes I mean some
page breaks get cancelled but some not, so this is inconvenient). I can't
put a minipage or something that surrounds the entire first block because
then I encounter the usual
\begin{environment1}
\begin{environment2}
\end{environment1}
\end{environment2}
problem, which is undesirable.
Any idea how to tackle this problem? I spent hours on it googling and I
ran out of ideas.