I've been playing for 36 years and still can't slap worth a damn. If it's something you really want to do, just keep at it.
This...but while acknowledging that it
might be a technique you're never able to get down to your satisfaction.
There are whole genres of music where not a single bass note on a single song is slapped. Don't get me wrong, slap is pretty cool, and the players who can do it well (read: musically/in context) can be enjoyable to listen to, but I wouldn't be hanging my hat and/or basing my "worth" as a player on that one technique if I were you.
Seek out quality instruction, practice as much as you can, and who knows, a year or two (or five) down the road, you might be a slap monster...but you might NOT be either...so practice being a "general" bass player too (regular fingerstyle, and/or playing with a pick)...work on being well-rounded, being able to play in time, and with good tone, in the genre(s) of music you enjoy/can see yourself playing (there's probably little point in learning to play country music, as an example, if you hate country music...)
I suspect part of the reason you have "hit the wall" is because you're a bedroom bassist...it might do you a world of good to start putting yourself out there, jamming with some like-minded people who might also hopefully become friends.
Finally, I'll reiterate what a lot of people have already said:
musical slumps/depressions are pretty normal. I have been playing since I was 16 (44 now) and have lost count of the times I have found myself in the doldrums...and also as some have already stated, sometimes the best remedy for this is simply to step away from bass for awhile...a week, three weeks, three months, whatever...it could make all the difference...

