<human>: Can you give me the gist of the text in a nutshell?

So I have generally avoided dating forever, but whenever I do get involved with a guy, there is a common pattern and I'm having trouble not letting it discourage me to the point that I avoid dating altogether. It just happened again so it's a very fresh frustration.

Pattern: Guy seems super interested but then after a couple months there is some abrupt/drastic shift and he disappears. Then I find out he started dating someone else quite quickly (which implies there was always another girl on the line).

Problem: I just assume/expect now that every guy will disappear and/or there's another girl in the picture. (Yes, I know it's not fair to predict the behavior of the next guy by the last few.) Further, since I am the common denominator in this pattern, I feel like I am the problem but I really don't think I am. Yes I like to take things slowly, but I've made that clear with each guy and they continue to show interest so I assume they're not just looking to hook up. 

Question: Going forward, how do I trust that a guy is genuinely interested and isn't going to just disappear, leaving me with no conclusion? Right now I feel like I'm always just an option that gets discarded. (I never pursue guys so all of my references are to guys who have pursued me.) As an introvert, it takes a lot for me to even put myself out there. I know I can't avoid dating forever if I actually want to meet a significant someone.
<bot>: How do I trust that a guy is genuinely interested and isn't going to just disappear, leaving me with no conclusion (when this has happened in all of my (albeit few) dating attempts)?