I couldn't count for toffee for years, and really only learned to do it as an adult, when I sat in on my kids' piano lessons. I found 1-2-3-4 etc just didn't work for me, nor 1 - & - 2 - & etc. - the work involved in thinking words got in the way of everything else I was having to do with the music.
The kids learned the ta-te sounds - taa, taa for minims, ta for crotchet, ta-te for quavers, and somehow that proved easier for me to internalise than words which have meanings. My recorder teacher also got me using metronomes regularly - I'll set one ticking, and make each beat a crotchet, practising the ta-te sounds with it, away from my instrument at first, then with it. Dividing beats up evenly is only a tiny step though; what's hard is playing off a beat, or long notes tied to very short ones, or shifts in rhythm. For rhythmically challenging pieces I'll often count a smaller time interval than the time signature tells - thus for four crotchets in a bar, I'll tend to count quavers, which makes it easier to sub-sub-divide beats; once that's internalised I do have to go back and rethink in crotchets though, as otherwise the stresses of the bar come out wrongly. When playing with others I tap my foot - I know purists will disapprove, saying it ought all to be internalised, but if it isn't then tapping is a good way to begin the process of internalising it. I have heard of musicians tweaking a big toe so their critics can't see

.
Don't lose heart over it. It was always my biggest stumbling block - and was bad enough when I was playing the piano, where at least the two hands can help each other out - it got much worse when I started playing a single-line-of-music instrument, but if I can manage to learn how to do it, you can too

. (I still stumble, I might add, but am confident enough to try playing with others now, and like you, have reached a relatively advanced level).