Making gravity constant (Flappy Bird)

I’m trying to make my character fall at constant rate (like flappy bird). The problem with gravity is, that it is an acceleration, therefore sometimes jumps are higher than other times, I want jumps to be equal. I tried just changing velocity in FixedUpdate but it doesn’t work as I want, it seems the pull is too strong and changing jump force doesn’t help, it looks glitchy.

    void Update()
    {
        if (Input.GetButtonDown("Jump"))
        {
            jump = true;
        }
    }

    void FixedUpdate()
    {
        rb.velocity = new Vector2(forwardVelocity, rb.velocity.y);

        if (jump)
        {
            rb.AddForce(Vector2.up * jumpForce, ForceMode2D.Impulse);
            jump = false;
        }
        else
        {
            rb.velocity = new Vector2(rb.velocity.x, -1f);
        }
    }

I don’t understand this line of code. It seems really counterproductive:

rb.velocity = new Vector2(rb.velocity.x, -1f);

I suggest deleting that line and just using Unity’s builtin gravity by enabling gravity on the Rigidbody. If you really want to do your own gravity all you need to do is call AddForce each frame with the gravity force (generally equal to gravity vector (0, -9.8, 0) * rb.mass).