Hmmm... That's a lot to absorb. Let's see...
You did well in high school but some scouts said you need more velocity. It would be logical to guess that you tried to increase your velocity but, in the process, you messed up something else.
Trying to throw softer messes you up. That's expected because you slow down and change your timing and end up throwing differently than how you've practiced.
You've tried different arm slots and that doesn't seem to help. This would seem to imply you should go back to the slot you used in high school. On the other hand, you say you're successful with any slot during practice but not during games. That would imply a mental issue.
Throwing from a low arm slot makes it difficult to throw inside due to the amount of tail you have. I'm not sure I'd consider this a problem. It's more just a matter of figuring out how to use it to your advantage.
Ok, my take on all of this is that there are multiple issues involved. I can only guess but it sounds like between the scouts telling you that you need more velocity and your telling yourself that you can't throw inside, you've messed some of your mechanics and you've lost your confidence. The mechanics can certainly be fixed. If you can, go back and work with those scouts/players you worked with in high school since it sounds like they got you throwing well. Or, post up video here and let us take a look. Once you've got your mechanics in order and fairly repeatable, that should make it easier to figure out the adjustment to make for throwing inside even with a lot of tail. Getting all of this in order will help with the confidence though that will take some time to get back as it depends on you preparing well enough that you truly feel well-prepared. It also depends on having some success. But those successes can happen in practice or in the bullpen.