Help me with Gemini situation?

Ack... So this Gemini has got into my head in a funny way. It's a situation that I can't quite wrap my head around. We've been talking for a few months as friends mostly but very flirtatious friends. He mentions wanting something more, but my bring insecure and dealing with several personal issues, said I wasnt quite ready.
Though I did say that, it didn't exactly mean that I didn't like him at all or want interested, I just needed some time.

After a week of silence, because of a miscommunication mishap (I thought I pissed him off), he mentions talking to someone else and developing feelings for them. This crushed me, especially because I had truly wanted to be in something with him. I didn't think he would move on so quickly. Thus, I thought I'd cut my losses, and withdraw not really wanting to compete for someones affection. Weeks after, we still talk, so flirt except everytime we flirted he mentioned, "oh probably shouldn't do that, you know.. Somebody else." Oof my heart strings both being tugged and mangled.

We still flirt despite his saying he was "committed" and I get even more confused. We talk more and he decides he's falling for me and "doesn't want to miss out on somebody he could care about." Anyway, sleep to months later (after he's decided to pursue things with me definitely) at some point we have a conversation about how hurt I'd felt over the constant flirting yet being "committed" to someone, going back and forth between two people, taking steps with her, and he mentioned that at the time the wanted to be fair to two people at the same time though I don't see how that is fair to involve the feelings of a person who expresses interest only to find out, that person had room for someone else. It wasn't choosing one over the other, he said, however I don't see it that way. It was a huge letdown. Either way, it all toppled my trust, the fact that he wanted to develop something but switched his mind last minute. And able to do so so easily. Just messes with my feelings heavily, and I tend to have heavy feelings for anyone I like anyway.

Can anyone clue me into his way of thinking, how it was "fair" to raise up someones hopes by flirting then saying there is someone else, then saying but I don't want to miss out. Very hard to keep someone waiting for you when that person also has to feel rejected in a way because of the introduction of another love interest. Clues anyone?

I can't speak for all, but before I settled down with my wife, I always kept my options open, dated different people and evaluated who I wanted to settle down with ( that's if he's at a point where he wants to settle down )

I was seeing 3 different girls at the time, 1 was an ex I was with for 2 years and was having a hard time breaking up with her. The other 2 girls I was seeing for about a month or 2. Then one day, I met the girl that eventually became my wife, and immediately broke it off with the other girls.

If he's telling you he wants to settle down, and there's no one else, that's a good thing, you have to let go that he was seeing someone else in the past, that will just annoy him. Be secure that he's with you now.