You give no inkling as to time scale here; are we talking days, weeks, months, years..? 'Plateauing' is a well-known phenomena in the learning process (not just instruments; it occurs in many other fields...). How to avoid it..? Difficult. What to do..? Work through it. For how long..? No idea, as it varies even for oneself. One or two tips, however...
1 - Practice little and often, rather than super-long sessions. Two bouts of fifteen minutes each, per day, are worth more than any two-hour stint.
2 - Little..? Did I say 'little'..? OK, but regularly. This is key; every day, with no exceptions.
3 - Start again: Pick up your very first method book, or first lesson notes, or whatever you started out with. Go through it, from the beginning, as if you're starting again. Do the exercises diligently (no cheating..!); it'll get you back, rapidly, to where you are now plus a bit more.
4 - Pick up your instrument as a 'leftie' (or 'rightie', if you play 'leftie'...); that how it felt when you began, and shows that progress has, indeed, been made.
5 - A bit more difficult, but essential... Arm yourself with a big bucket of Patience; all players, at all levels, need this, and need to fill it up regularly. Learning is a Long Game, and never finished. Just when you think you know it all, you realise that you don't. This is Normal.
6 - Set yourself achievable goals (targets...). A song to learn, a technique to attempt, a genre to bring on board... Give yourself a decent time scale for it, and add it to your practice schedule. Go through the basics, go through your next lessons, then have a go towards this target. Every day in short sessions, going back now and again over older stuff. It'll work; we've all been there.
Now for my tried and tested 'words of encouragement'...
'It's the first forty years that are the hardest, after which things sometimes tend to get slightly better.'