I’ve been having trouble getting into the 1.5 mount. No matter what, the string I’m supposed to go around just rejects the yoyo gap. I can double or nothing with ease now, but even when I try to give some slack from my non throw hand, it doesn’t want to go around my finger.
I’m stuck too because I can’t do a lot of the advanced tricks without getting this mount down and I have some of those other elements already down. Anyone’s help would be great!
When i was learning it, what I did was turn that loop that is around you NTH pointer finger counter clockwise (90 degrees), so that that loop is on its side. Then just move the yoyo slowly and carefully into that string. Now, that may seem slow when you go an practice that, but eventually you’ll get faster and faster until finally, you can get into one as smooth as silk. I hope that made sense, and helped you out! 
That did help a lot, I can kinda land the string now, just gotta make it more fluid now since my swing around is still a bit twitchy.
I found it easier if you do what brandon1 says, but think of it as going into a sideways split-bottom mount
When I had trouble hitting that mount, I would land on a Trapeeze, then pop the yoyo up like Kwijibo, I would shove my finger with my T.H. into the string right next to the yoyo. This way, the yoyo was forcibly pushed into the string, then I let it fall into a 1.5 Mount. Eventually, landing it straight from a Breakaway came as second nature.