Values in editor script resetting

In my editor script, values get reset to default when entering play mode or deselecting the object. I've seen this question many times, but none of the answers given seem to work for me. [SerializeField] doesn't seem to work, and it doesn't make a difference if its public or private. This is the part of my code that does all the value setting:

 [CustomEditor(typeof(BezierPath))]
 public class BezierPathEditor : Editor {
 
     [SerializeField] private Color lineColorStart = Color.red;
     [SerializeField] private Color lineColorEnd = Color.blue;
     [SerializeField] private bool drawNodeHandles = true;
     [SerializeField] private bool disjoint = false;
     [SerializeField] private float lineResolution = 0.1f;

     ...

     public override void OnInspectorGUI()
     {
         DrawDefaultInspector();
 
         EditorGUIUtility.LookLikeControls();
         EditorGUILayout.Separator();
 
         //Draw transform handles option
         drawNodeHandles = EditorGUILayout.Toggle("Show Handles:", drawNodeHandles);
 
         //Disjoint Handles option
         disjoint = EditorGUILayout.Toggle("Disjoint Handles:", disjoint);
         EditorGUILayout.Separator();
 
         //Line resolution
         lineResolution = EditorGUILayout.Slider("Curve Preview Resolution:", lineResolution, 0.01f, 1);
 
         EditorGUILayout.Separator();
 
         //Line Color Picker
         Rect colorRectStart = EditorGUILayout.BeginHorizontal();
         colorRectStart.x = 115;
         colorRectStart.width = 125;
         colorRectStart.height = 15;
         GUILayout.Label("Start Line Color:", "BoldLabel");
         lineColorStart = EditorGUI.ColorField(colorRectStart, lineColorStart);
         EditorGUILayout.EndHorizontal();
 
         Rect colorRectEnd = EditorGUILayout.BeginHorizontal();
         colorRectEnd.x = 115;
         colorRectEnd.width = 125;
         colorRectEnd.height = 15;
         GUILayout.Label("End Line Color:", "BoldLabel");
         lineColorEnd = EditorGUI.ColorField(colorRectEnd, lineColorEnd);
         EditorGUILayout.EndHorizontal();

You'll notice that i use DrawDefaultInspector() to show a few variables from the regular script this editor is used for, but those values are NOT reset. Just to make sure its clear, the values that are being reset are ONLY being used by the editor.