The answer is yes and no (don't you just love the clarity you get here on talk to God?)
If you're acknowledging your mistakes, asking God to help you not repeat them, and taking the necessary steps you need to make amends to anyone you hurt by your actions or words, then according to Judaism, this takes care of the problem, and the mistakes are cancelled out spiritually.
NOTE ONE VERY IMPORTANT CAVEAT: For mistakes where you hurt someone else, you absolutely have to do your best to make amends to the person in question, say sorry, and to try to fix whatever it is you broke, before the mistake is cancelled out, spiritually.
If you skip this stage, the 'mistake' is still live spiritually, and needs to be dealt with. Clearly, the bigger mistake, the more effort it's going to take to fix the mess that was made, but it's definitely always possible to fix it.
A famous Jewish mystic who lived around 200 years' ago, called Rebbe Nachman of Breslev, expressed it this way:
"If you believe you can break it, then believe you can fix it."
Which brings me neatly on to:
If you're praying for forgiveness with no intention of trying to fix the mess you made (as set out above) or if you fully intend to repeat the 'mistake' again in the future, then your prayers probably won't cancel out the mistakes.
If it's something that's very hard for you to avoid doing again, eg, like not losing your temper, not overeating, not getting drunk next time you meet your friends for a night out, then the way to handle it is this:
Admit that you can't really prevent yourself from making these mistakes again in the future, but ask God for help, because you really don't WANT to be doing this stuff any more, regardless of what actually happens.
In Judaism, your desire to do good is the main thing that counts. If you:
- Genuinely want to avoid making the same mistake again
- Do your best to fix the problems you made and to make amends for hurt you caused others, and
- Continue to ask God to help you behave better next time
Then spiritually, you're covered.