- write the count out in pencil on your score, so you're not trying to remember what comes after "three" when you're trying to play and you aren't decoding what pitch goes on which count in realtime.
- practice just the rhythm clapping it, chanting it, playing it on the tonic note.
- practice counting aloud with a recording of it to get a feel for it.
- count only one: as a practice exercise, practice only saying "one" at the downbeats and no other counting to learn the association of where the measures start with the habit of saying "one!" This may make it easier when you then bring the rest of the count in.
- exaggerate the emphasis: as a practice exercise, play with a really exaggerated marcato rhythm until you're getting everything in the right place in time. You can be cantabile later.
Finally, let me say, I think a lot of problems junior musicians have with counting and rhythmic accuracy are at least exacerbated, if not caused, by a bad attitude about rhythm, that playing the "right notes" in the right order is more important than playing them in the right moment.
The most important aspect of maintaining your rhythm is NOT counting. The most important aspect of maintaining your rhythm is knowing the feeling of your rhythm. Counting is a tool, a means to that end. It's the scaffold with which you build the building, but it's not the building. Or perhaps more accurately, it's the measuring tape that keeps you honest as you put the footings of your pillars down. If you're having trouble getting your footings to match with the measuring tape, maybe that's because you're placing them based on mistaken estimates of where they go.
The point of counting is to give you the feel for how the piece goes, how its impulses fall, accurately. If the counting is fighting you, maybe the issue is you need to get the feel of the rhythm into your body, understanding it not as a semantic thing ("two half notes followed by 4 eighths and a half note") but a sensation ("DUM! DUM! DADADADA-DUM!")
Rhythm is not an arbitrary series of durations: it is the other main dimension of beauty in music. If melody is the part of music that sings, rhythm is the part of music that dances. Those series of durations were chosen for a reason, and have meaning and expressiveness. The point of counting is to attempt to reveal that to you, so that you can reveal them to the audience. Get into them, and I think you will begin to find them compelling, to find they take on a life of their own.