Just find simple combos you can do in match, then slowly add on to them.
It's natural to be able to do the combo in training mode when you can do it at your own pace, then have a harder time when you're in the heat of a battle. It happens with every combo and becomes second nature over time.
But make sure you can REALLY do it in training mode. If you try 10 times, and only do it successfully once, then you haven't got it down yet. It needs to be ingrained in your muscle memory so you can do it without thinking. If you can do it ten times in a row without messing up, then you can do it. The reason you drop a combo a lot of the time is because you have to think about it consciously. You will land a hit, then go "Oh, what do I do now? Right, it's ____" but by then it's too late. It needs to become reflexive.