think of a map that has 1/1, 1/2, and 1/4s,
and you singletap all the 1/1s and 1/2s, but alternate the 1/4s(this what people would call a singletap playstyle);
when you do that, you're essentially keeping a 1/2 rhythm with your dominant hand, and in the 1/4 parts you're using your other hand to do the offbeat. this playstyle allows you to understand and keep the rhythm of the map quite easily. however with singletap you are forced to use one of your hands much more.
now when you're alternating also the 1/2s and possibly 1/1s, which you aren't used to doing, you have to shift speeds according to the density of the notes. this means that both your main hand and off hand will be constantly shifting speeds.
obviously keeping a rhythm with this playstyle is significantly harder.
so your issue seems to be that you're not used to shifting speeds, and my recommendation is to play slower bpms, as slow as necessary, and try to use the new playstyle in those maps. practicing in high bpm maps will be a lot harder, your brain doesn't comprehend this method enough to do it at higher speeds(probable cause). you are better off getting used to alternating in low bpm maps, since these will make it very clear what you need to do and when you mess up you'll understand better what you are doing wrong.
and of course, it takes time and patience to learn something new, so don't curse yourself if you're not playing perfectly with semi-alt overnight.