If it's the right thing to do, then why does it hurt so much?

Well, I had to have "the talk" with my partner of 3 years yesterday and she took it pretty well, all things considered. We both agreed that we hadn't been happy for about a year. In fairness, we have been through a lot together and she has been very supportive through illness and family bereavement, which all happened within the first two years of our relationship. We do have a deep connection on many levels, but there are numerous fundamental differences that I feel won't make it work long term. Saying that, this has been the deepest relationship I have ever had and I'm very worried that I could be making a mistake...? Perhaps it's normal to feel like this...but it really hurts already and feels like such a wrench. It's difficult to describe, but we've built our worlds around each other and we thought that we would be together for the long haul. It's such a shock to be in this situation. I hate hurting someone's feelings. I know she has negative traits but that doesn't detract from the fact that she is a supportive, caring person with a good heart. I feel utterly devastated right now and again, not really sure if this isn't something that could be saved.....

As I type this, I'm thinking of all the possibilities - perhaps a trial separation, but would that be putting off the inevitable?

I'm 42, which isn't exactly old, but I'm wondering if I'll ever find another relationship (this deep)? Am I making a terrible mistake and that perhaps things can't be sorted out somehow?

Am I actually letting something irreplacable slip through my fingers..?

How many more times can I go through this?!

It sounds to me that you have made and are making a terrible mistake.

No relationship will ever be perfect and over time the passion will subside but if there is a scared commitment and moral basis you may have something lasting and if so that is worth more than gold.

You may never find anything as deep again.

I think you should seriously consider reconciling before it is too late.

If it's the right thing to do, then why does it hurt so much?

Because when you break something, it hurts. The rightness or wrongness of breaking something doesn't change that.

However, if you break something badly, or wrongly, then the level of pain is likely to be worse.

but there are numerous fundamental differences that I feel won't make it work long term.

You didn't say what they were in your post. I guess either that means they're very private, or you think they're not important. But you did say they were "fundamental" and "numerous".

Since they are, why are you posting here? It sounds like you have your answer.

I would be more interested in what you think and why, than what you feel. Especially right now since you've just put your heart in a blender and switched it on.

I'm 42, which isn't exactly old, but I'm wondering if I'll ever find another relationship (this deep)?

No one can say you will. But I will say you can.

The depth of a relationship is a function of time as well as connection. A shallow connection can become deep over time. A bad connection does not become good over time unless things are done to change it.

Do you have a shallow connection or a bad connection with her?

Am I making a terrible mistake and that perhaps things can't be sorted out somehow?

Did you try sorting things out?

Am I actually letting something irreplacable slip through my fingers..?

Yes because everyone is different. No because everyone can be replaced by someone different, maybe better, maybe worse. Finding a replacement, especially a good one, can be difficult and time-consuming. At age 42 I'd say you got another 5-10 years to find a good one depending on a few other things. If your name is Jack Nicholson then you probably got another 50 years before your options become significantly more limited.

How many more times can I go through this?!

That's a dumb question.

You dumped her and you haven't told us why. Don't expect useful advice.

If she's negative, or a negative influence on you, then dump her, suck up the bad feelings, and get on with your life. If you're bad for each other, at least try and discuss it honestly and agree to do what it takes to be good for each other, or agree to break up. If you're bad for her, be honest and tell her, and try and give her the choice to dump you or not but don't be an ass and try to manipulate her into dumping you.

but there are numerous fundamental differences that I feel won't make it work long term.

Would be helpful to know what you think those fundamental differences are if you are looking for feedback and advice.