[38/m] We broke up by mutual agreement. She [22/f] wants to stay “best friends”. Good or bad idea?

TL;DR at bottom, per the rules.

Hello internet friends. I have a lot more of you than I do friends IRL, so I beseech thee for advice. Yes, it's a brand new account, just minutes old. I created it just for this question, because I have privacy concerns. For one, she is a Redditor too. Because of this, it may be difficult for me to answer any questions with any detail. But I'll do my best, I promise.

We were together almost 4 years. For most of that time, we both felt everything was perfect and we had our entire future planned out, and that definitely included marriage. Over the last 3 months, our age difference became an issue. We didn't have a problem with it; but it created the problem.

I began to feel more and more that she was in it for the sex more than anything, and I was in it for the long-term romantic/permanent relationship/life-partner thing. I was really starting to feel the differences in our emotional maturity. Don't get me wrong about the sex: it was perfect, but I like more than sex. We overcame it for the longest time because I act a lot younger than my peers, and she's very mature at times, but little by little, I saw the problems creep in.

By mutual agreement, we have broken off the relationship. At her request, we are remaining in contact. She wants to stay “best friends”. This hurts me because I was in love with her and wanted to be with her the rest of my life. I'm afraid that continued contact with her will only cause me more pain and make “getting over her” take that much longer. I do like the idea of still seeing her, but knowing we will most likely never be more than friends again hurts a lot right now. Will it hurt less in time?

Please give your opinions, and if you have similar experiences, tell me if staying “best friends” was a good or bad thing for you?

I'm going to bed soon, so I won't be here to answer questions, but I will check back often and do my best to provide answers as long as they aren't too detailed.

One more thing: Please don't give me crap about dating an 18-year old when I was 34. I didn't come here for that, I came here for advice.

TL;DR: Should you stay “best friends” with your ex?

Thank you in advance.