It sounds like you are not varying your play enough.

The only real mistake HU is to fold your SB. And even that is OK if you do it 5% of the time just to be random.

If the blinds are small, and your are not going to fold (i.e., you are SB or have anything at all in the BB), then you can just raise/call in a random fashion.

If you miss the flop - bet sometimes, check other times. If you are raised big fold, unless he starts doing this repeatedly. If you are raised small, call and try to check it down.

If you hit the flop... bet sometimes, check-call sometimes, check-raise sometimes. Whatever you do, don't get into a pattern, UNLESS you are concioiusly setting a pattern only to break it when the blinds go up.

If the blinds are big, you can just push any 2 from the SB. It's +EV, especially if you (still) think he has your number and can out play you preflop. If you do this twice in a row.

HU is so different because with only 2 hands it's extremely unlikley that you are dominated (or are dominating). Even with 22 - you are only dominated like 1 in 17 times.

With a random X2 (most likely to be dominated) - the odds he has an XY with Y>2 are 3/50*46/49 * 2 = about 1 in 9.