So, let's start from the very beginning.
You started chatting up a girl through the internet and you hit it off, had a good talk, possibly a spark. Which is a great starting point. Then, from what it looks like to me, you just kept talking and talking online, without actually moving it anywhere and just passively waiting for 'something' to happen, 'somehow', while being stuck in the same internet chatting cycle. Because you did not have the balls to move it further, ask her out and get physical and was afraid of screwing up what you already had at that point (while, in reality you had nothing going on, sorry to tell you that). I think it's fair to assume she innitially liked you and was imagining you as a possible lover (well she said it herself later). You are saying 'Once she suggested if we could talk this much and all seems to be good then we should meet.' at this point, i'm pretty certain that she was expecting you to make a move and ask her out, which you didn't. She was getting tired of waiting for you to man up, but still curious if there could be something between you, so she was willing to help you out there a little bit by throwing you a little stick and actually inviting YOU out. You said that you panicked a bit, started overthinking everything, doubting yourself and trust me, women can smell this kind of anxiety from miles even from text so i would say that was the final nail in the coffin for her. My guess would be that is also why she stood you up when you finally grew a pair and agreed to meet, which unfortunatelly was too late. At this point you started to feel that you are losing her and started to act more and more desperate, you probably confessed your feeling to her (never ever ever do that without getting physical with a girl prior to that and never ever ever ever ever do that through online chat. it is creepy as fuck). In a situation when you havent met before and you did this, she probably assumed (probably correctly) that you dont meet any girls and that you are desperate as hell and continued talking to you out of sheer sympathy. Which i think is also why she later suggested you would stop talking.
Eeeeeeeeeeeeeeeeeeverything else you are mentioning later is really irrelevant, trust me.
Her being older - bullshit. My roommate is 30 and she is dating a 19 years old guy and they are hitting it off perfectly. 5 years of age truly do not matter if there is some attraction. Her thinking about starting a family? With a guy she never met face to face? Please. At this point, she was just trying not to hurt you and started giving you all these 'rational' arguments why it wouldn't work out between you, while the only reason here was that she simply lost the initial interest for you, because you never grew a pair of balls and asked her out.
It's not even mental health issues, trust me this shit is something that every guy is going through at certain point in his life. At this point you are trying to blame it on mental health, on personality differences, on the situation that was not too convinient, while you are only trying to run away from something that seems kinda obvious to me from your post, you simply lack confidence. And honestly the only solution here is to work on your confidence and self image. Its a hard work, but noone else can do this for you.
Let me get a little deeper here.
If you are trying to meet a girl on the internet, online chatting can only get you to a certain point. You can project some things about yourself, you can see what kind of person she is (whether she is intelligent, what she likes etc) and it can be a great starting point. What you need to do, is to spark an attraction in her. That tickly thing in the belly that makes you think about the other person all the time and make you want to rip their clothes off when you meet. The tricky thing is that attraction works in a completely different way for girls than it does for us. IT IS NOT POSSIBLE TO GET A GIRL ATRACTED TO YOU THROUGH THE INTERNET, write that down with bold letters somewhere. To get a girl attracted, you need to interact face to face and without getting her attracted to you, nothing is going to happen, ever. Attraction is ignited by eye contact, body language, touch, the way you talk, the way you smell etc, not through text on the screen of a phone, no matter how funny or interesting or relatable it might be. If you never meet, you wont get her attracted, and nothing will happen, there is absolutely no way around that and girls are very well aware of that. Also, to get to this point, you have a certain time to do this. From the point when you start talking online, you get several weeks, one month at the top to make a move and if you won't, she will either assume you simply talk to her because you see her as a friend and you like talking to her, or figure out that you don't have the balls to make a move and loses all the initial interest (your case). Either way, nothing is never ever going to happen from this point. Why? Because women want to date a man, a masculine person with two balls and a straight back. If you are hesitating for weeks and acting like a pussy (and again, girls can and will detect this quickly), they will realize you are not confident with your decisions, you are scared to take a risk and dating you would most likely be super unexciting and that she would be the one wearing pants in the relationship. By hesitating, being passive and not moving it forward, you gave her all the power over yourself in the situation and that is something you never want to do. You must be the man, you must be the one leading the interaction forwards.
Good news - it can be fixed. Might not be easy, might not be quick, but it works perfectly. You need to get confident. Easier said than done, i know... Well, there are tons of great books on how to work on your confidence out there and this post is getting super fucking long anyway, so i dont want to go into that

But... Next time you start talking to a girl online and hit it off, promise to yourself - if everything goes well, you will just ask her out withing 2-3 weeks of having good chats. If you won't, this will happen again, i guarantee you that. She won't bite you, she won't laugh at you, stock market wont collapse, world wont start spinning the other way and there is a good chance she will gladly agree because she was expecting it (it could have been this case we are talking about).
And this girl, well, stop thinking about her and stop talking to her as quick as its possible, get rid of her from your social media, delete her number. Unless you want to get tangled in your obsession and overthinking even deeper.