Over time I have leveled up and want to contribute. This is a small guide how you could improve.
Try to train only one combo at a time on a dummy with auto block. (eg. c.LP, c.HP, HP.Srk)
Once you have a feeling for the combo try to alternate stuff. Try your combo -> wakeup -> Srk FADC Ultra -> wakeup -> Crossup combo. This way you force yourself to adapt your timings.
If you feel secure, try to punish different moves with your combo. (eg. use Cammy and record LK.Spiral and spam Cannon Spike). Block and punish with your combo. Alternate the recorded moves and characters.
Do not question yourself. Think: I will land my combo for sure. Even if you fail this is better than doubting your abilities.
If you feel comfortable doing everything try to hit confirm. If your opponent blocks your c.LP try to switch HP.Srk with HP.Hdk (c.LP, c.HP, HP.Hdk) This way he cannot punish you.
Try to hit training mode every day for about 10-30 minutes. It is better to train your combos 6 x 10 minutes than 1 x 1 hour.
Do not lie to yourself: Count how often you can execute a certain combo without failure. I consider a combo usable in fight if you can execute it 7-10 times in a row on a regular basis. In a fight your adrenalin will raise and your execution/timing will get worse.
Use a computer opponent to get a feeling for punishing.
The computer is no good on the long run, but consider this sparring instead of fighting.
Try to block as much as possible and only punish with your trained combos or use cross ups.
Use custom constraints to make this fun. (e.g. no jumping or no fireballs or no anti air shoryuken during the whole game)
WARNING: The computer will not adapt to your playing style. Real opponents will crush you if you do the same thing over and over. Once you are in a real fight just throw in grabs, overheads and random shoryuken.
Once you master a certain combos new ones are way easier to learn.
Try simmilar stuff (e.g. c.MP, c.HP, HP.Tatsu)
Try punishing with your combo, even if other options are easier to execute or do more damage. (e.g. If opponent wiffs HP.Srk, instead of Solarplexus, HP.Srk use your combo)
If you miss your combo, consider it a success. Do not force yourself to make it right ASAP, because then you get predictable and also do not stop using it. Just throw it in once in a while.
[*]Even if you lose, do not go back to old bad habits.
Remember: You want to become better on the long run.
Try to sleep enough and do not train when you are tired
Train only when you are sober
Do not get frustrated if you cannot execute your combos properly. Just try it again tomorrow.
Try to measure your success against the same buddy in offline tournaments.
[*]It’s easier to land your combos in matchups you know by heart. (e.g. I am confident in Ryu vs. Juri, but I suck against Cammy)