I'd have to agree with April. I don't think that's fair to yourself to say "I'll never give up. The door will always be open for my X".
I'm not saying, once you're divorced or going down that road that you should walk away and never look back, but you need to have a life. You need to be complete, happy, and content with what you do have. If the XS chooses to come back then you can decide if they are a person that can enhance your life or not, because that's what marriage is, being more than the sum of the parts.
But we each have to reach the point of moving on or not at our own pace. It's a stage in the divorce process. But there is life after divorce and there is love after divorce if you are willing to grasp it. But it won't come if you don't heal. Your XS won't even come back to you unless you heal. Honestly, who really wants an incomplete person? Even your WAS/XS can see that you aren't strong enough, complete, or happy without them. No one wants the burden of completing another person's life. It's hard enough taking care of your own.
You don't have to lock the door, but it's my opinion that you do have to close it and walk away from it. If your XS has a desire to try again, they can knock and talk to you from outside the door. It's not fair to just let them in, no questions asked, as though it was entirely your fault that they left and they somehow don't have to prove themself to you.
In the end, it's not the years in your life that count. It's the life in your years. Abraham Lincoln
It is hard to fail, but it is worse never to have tried to succeed. Theodore Roosevelt