Metronome, definitely, or better still, a drum machine, one that has at least one boom among the clicks. But also one that wont let you waste time trying to program a new beat into it.
Ghost strumming also helps a lot -strum in the air on every 2nd beat for example.
Playing along with your favourite song vid can help to get your whole body moving to the beat, also standing up and swaying to the beat can help too. Bend your knees!
Dance lessons...
Read a text while simple strumming to the metronome. You’re trying to separate the words in your head from timing physical muscle memory. That should be automatic.
Finally remember that if the metronome irritates you, it’s partly the sharp clack (that you need for accuracy), but a big part of it is that you are following it. In time you will use it less, every fourth beat say, to confirm you’re in the right place and then even longer stretches for example when taking a solo instrumental, at the end, because of focus on the notes you can sometimes lose the timing. The others around you will help bring you back.
Another is to swing the pick or hand an exact amount for each beat, same pressure on the strings, same angle of the pick to the strings, planning to get back to the beginning of the swing at exactly the right time, and don't let left hand movements accidentally change the right hand swing.
Also count 1,2,
3,4 out loud all the time (if the song’s in 4/4 time) and remember that other people’s chord changes and accents give indications of where the measures are. Later you wont have to count four beats every measure, you will only hear the
3 -this helps a lot.
Listen to music and count out loud. Know that there’s a slow down option, 50% I believe, it’s an icon below the window on YouTube, but that some bands, even some good ones can slow down and speed up a lot.