Why is it so hard for me to love?!!?

[497]

You could be aromantic, or asexual, or both, as a result of what happened to your father.
Aromantic: https://www.google.com/search?q=define+a...2&ie=UTF-8
Asexual: https://www.google.com/search?q=define+a...2&ie=UTF-8 (Look at the second definition)
Yes, you can be asexual, but want romantic connections, or want sexual connections, but nothing romantic.
Thrid thing: You could def just have trust issues, and not be any of the above. Some weird ear cancer is quite strange, but I'm sure if he promised he'd never leave you, then he died fighting cancer the best he could. Would he intentionally give up when he promised his little girl (who isn't so little anymore?) he wouldn't leave? Your dad is probably still with you, in soul, or spirit, if he believed in that stuff.
To trust again, just take little baby steps. It will most definitely be uncomfortable, but if it is TOO uncomfortable, you can stop or backtrack at anytime. Just make sure to go at a slow pace you're comfortable with, and tell people who will be involved in your regaining-trust thing, and have them RESPECT you and your choices in this, be patient, and tell them to do likewise.