You gotta learn what parts of a combo are mashable and what parts aren't. There will be points you want to be slow and accurate, other times use a rhythm, and others mash. Every character is different with this too. Almost all characters can do MMHS in the air, but may require timing differences. Some you want to wait till your at the apex of the jump to start, others early, some may want you to slowly input parts. On ground combos I've learned with Trish and Morrigan to always hit LM quickly, then hit H when I hit confirm. If I try to L and with M wont connect, but H I can wait for. So play around with timings and look at videos to grasp when they're inputting.
Also consider making your own combos. Trish in the guide has LMHS, j.M 2H 214L, double jump, j.MMH 2H 236L 236AA. I fuck up the 214L part all the time so I cam up with: LMHS, j.M 2H, land, LMHS, j.MMH 2H 236L 236AA. It's nearly the same damage and meter, but it's easier, more consistent, and the timing is generally do things as fast as possible rather than play with the time intervals.
Also pick up the guide, it's really worth it.
On another note...fuck sentinal still. Although Ammy j.H/2.H spam is fucking funny on him.