Custom gravity and Velocity

Unity Engine
1

Hi, I am trying to achieve realistic movements on my object for which I have been writing a custom gravity (with gravity Switched-Off on the RigidBody Component) and velocity script, but my gravity does not act properly. Even if I Switch-On the gravity in the RigidBody Component, My gravity and jump do not work. My gravity(Scripted or from the editor) and Jump seems to work properly when I do not implement Movements. I have also tried reducing Drag below zero and yet my gravity and jump do not work. Here are the Scripts -

Physics.cs -

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
namespace CustomPhysics
{
     public class PhysicsClass : MonoBehaviour
     {
         private static PhysicsClass _physicsInstance;
         public static PhysicsClass PhysicsInstance
         {
             get
             {
                 if (_physicsInstance == null)
                 {
                     _physicsInstance = GameObject.FindObjectOfType<PhysicsClass>();
                 }
                 return _physicsInstance;
             }
         }
         //gravity fields
         private Vector3 _gravityVector;
         private float _jumpAcceleration;
         private float accelerationDueToGravity;
         //movement fields
         private float moveAcceleration;
         private float friction;
         private Vector3 _moveVector;
         private void Awake()
         {
             moveAcceleration = 5;
             friction = 1.5f;
             accelerationDueToGravity = 9.8f;
         }
         public float JumpAcceleration
         {
             get
             {
                 _jumpAcceleration = 3;
                 return _jumpAcceleration;
             }
         }
         public Vector3 GravityVector
         {
             get
             {
                 _gravityVector = Vector3.down * accelerationDueToGravity;
                 return _gravityVector;
             }
         }
         public Vector3 MoveVector
         {
             get
             {
                 float x = Input.GetAxis("Horizontal");
                 float y = Input.GetAxis("Vertical");
                 _moveVector = (new Vector3(x, 0, y) * moveAcceleration) / friction;
                 return _moveVector;
             }
         }
     }
}

GravityAndVelocity.cs-

 using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using static CustomPhysics.PhysicsClass;
public class GravityAndVelocity: MonoBehaviour
{
     private Rigidbody RB;
     private Transform objectTransform;
     private float heightFactor;
     private void Start()
     {
         RB = gameObject.GetComponent<Rigidbody>();
         objectTransform = gameObject.GetComponent<Transform>();
         heightFactor = 1.75f;
     }
     private void FixedUpdate()
     {
         //RB.drag = -100;
         if (objectTransform.position.y > 0.1f)
         {
             gravity();
             if (Input.GetKey(KeyCode.Space) && objectTransform.position.y < heightFactor)
             {
                 RB.velocity = Vector3.up * PhysicsInstance.JumpAcceleration; // * Time.time;
             }
         }
         RB.velocity = (PhysicsInstance.MoveVector);
     }
     void gravity()
     {
         RB.AddForce(PhysicsInstance.GravityVector);
     }
}

So, What i have done wrong? please Kindly, Explain.

2

Answer- when space pressed - downforce += upwardforce;