If you're having trouble landing combos in a match that you can do consistently in training mode, play arcade mode. People tend to disregard the value of arcade mode, because computers don't act like real players, but it can definitely be useful if you focus on the right things. Hit confirming, doing combos in a real match, spacing, converting off of weird situations like trades... Also, if you're not already familiar with the entire cast, it can help you learn what each of the characters can do.