Unwanted space after section title when using custom command

I have been using LaTeX for years now and I am usually able to answer my own questions myself or thanks to stack exchange. Until now. I have this strange vertical space appearing after the title of the section (see picture). It actually doesn't happen when I don't use my custom command \fillspace. But in a later subsection (not shown here), it doesn't happen even though I use the same command \fillspace, so it doesn't seem to be a regular problem. Strangely enough, it doesn't happen in the minimal sample code I give you with this question (the original code is for a 25-pages-long course...). The other command I created and used is \slv (which does the same as \\ but prevents the recurrent "no line to end here" error). I suspect that there is a subtle bug with my new commands but I just can't figure it out.

Do you know how I could remove this useless blank space? I can share the whole code with you, if needed.

Thanks in advance for any answer,

Bests,

Henri

\documentclass[a4paper, 12pt]{book}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern} % load a font with all the characters
\usepackage{amsmath}
\usepackage{textcomp}
\usepackage{graphicx}
\usepackage{float}
\usepackage{stmaryrd}
\usepackage[margin=0.8in]{geometry}
\usepackage{amsfonts}
\usepackage{appendix}
\usepackage{wasysym}
\usepackage{pifont}
\newcommand{\slv}{\leavevmode\\}
\newcommand{\exo}{\vspace{0.1cm}\slv \ding{46}\textbf{\underline{Exercises}}: \\ \vspace{0.1cm}\slv }
\newcommand{\exm}{\vspace{0.1cm}\slv $\triangleright$ \textit{\underline{Examples}}: \\ \vspace{0.1cm}\slv }
\newcommand{\rem}{ \vspace{0.1cm}\slv $\bell$ \textbf{\underline{Remarks:}} }
\newcommand{\fillspace}[1]{\vspace{#1cm}\slv \checkmark\slv \vspace{#1cm}\slv}
\begin{document}

blablabla

\section{Second degree equations}
Among the equations that are still easily solvable are the second degree equations, which contain at least one second degree term and can generally be written\slv 
\fillspace{0.5}
These equations have between zero and two real solutions, as we will see later.\\
\exm 
1) Let us start from the equation $5x^2+3x-8=0$. We have\\
\vspace{1cm}\\
2) For the equation $2y^2+9=0$, we have\\
\vspace{1cm}\\
3) If we have something like $z^2-7z-3=5z^2-12$, we can transform the equation into\\
\vspace{1cm}\\
and thus we have\\blablabla...

\end{document}

Ok, thanks for the help. I have to admit that I never used the command \par before, which is weird considering how useful it seems to be. I always overused the double backslash , apparently. I also modified my custom commands using the \paragraph structure, which is probably better. In my own answer that I will write below is my new code. It appears there are still places where I need to use the double backslash, such as in the tabular environment. What do you think?

@HenriDegueldre yes \\ ends tabular and math alignment rows, but that's really a different command altogether, but it should almost never be used in running text. \par is just the same as a blank line (a blank line generates a \par token which is why it ends a paragraph, but in macros it is sometimes easier to write \par than use a blank line)

Ok thanks a lot! This will definitely improve my TeX coding.