Knight, I can relate to some of what you write...
Well, you know what they say: hope for the best and prepare for the worst?
I've been through some 'abandonment-related' issues and experiences too... Some people just die, or leave, or go different paths, or whatever... It's basically a fact of life... It's easier and better to just accept it... (It's not easy, it can be done though...)
For me, the solution is to always have people you can turn to - a few people as friends, a few forums to go to, a few possible mentors to go to...
In the past, a friend didn't like it, that I didn't put my whole attention to her as in 'best friends', but for me it was logical to be friendly with other people too...
Learning to get to know people and make friends is a skill.. And it's a transferrable skill, you can use it in all aspects of life.. There are books and articles and websites on this..
You are talking about someone romantically interesting to you... I do hope it works allright for the two of you.. (and you got some good tips from others above) It's important to get to know each other well first, your hopes and expectations, wishes and dreams for life... to see if you are compatible or not..
Know that it's good to have friends and other interests too, and 'backup plans' (even if it's as simple as 'watch film X or read book YZ' (several times)....

) so your whole life and future doesn't depend on 'one person only'... It's good that you are learning to love!!
Even if this doesn't happen in an exact same way as you'd wish, it can still be 'practice' for when you meet someone you are truly compatible with... You may know better what you are really looking for in a partner/relationship etc.
Some fear is normal, even people who later had very good relationships or got married were sometimes insecure or doubtful at first.. It's also good to listen to your intuition and your values a bit, and observe the other person well, to see if this person would really be someone you could have a good life with...