So I am trying to make my character regenerate health but reset the time it takes to regen when they are hit. I'm lost and have been at it for hours thanks in advance

You probably want to do something like this, you need to keep track of when your countdown was last triggered

 using UnityEngine;
 using UnityEngine.UI;
 
 public class Player_Health: MonoBehaviour
 {
     private float start_Health;
     private float current_health;
     public Image HealthBar;
     private float regen;
     private float regenerate = 10f;
     private float lastTime; //This will keep track of when you last regenerated
 
     // Use this for initialization
     void Start()
     {
         start_Health = 100;
         current_health = start_Health;
         lastTime = Time.time;
     }
     public void OnTriggerEnter(Collider col)
     {
         if (col.tag == "Enemy")
         {
             current_health -= 10;
             Debug.Log(current_health);
             HealthBar.fillAmount = current_health / start_Health;
             lastTime = Time.time;//this will reset your delay
         }
     }
     public void regenerating()
     {
         if ((Time.time > lastTime + regenerate) && (current_health < start_Health))//this will trigger when the time is greater than lastTime plus your delay
         {
             current_health += 10;
             Debug.Log(current_health);
             HealthBar.fillAmount = current_health / start_Health;
             lastTime = Time.time;//this will reset your delay
         }
     }
 
     // Update is called once per frame
     void Update()
     {
         regenerating();
     }
 }