Set aside a little time from your schedule and think through all the things that went well this year and all the things that could be improved upon. Don't rush this - think deeply and take time to reflect on everything from training to race execution to your mental approach to relationships to nutrition.
Identify everything that you feel is holding you back and write down an action to improve that area. Make sure your list is specific to you and areas you need to improve - if you have excellent stamina there's no need to put "complete my long steady swim every Wednesday without fail". Sure you might want to do that anyway but it's not a weakness that needs addressing.
Also make your list very specific - for instance don't say "cut back on the booze", that's to vague and easy to dodge. Instead write "I'm only going to drink alcohol at the weekend and never more than two glasses of wine".
Some other examples:
- I'm a classic over-trainer, if I'm feeling tired after warming up I'm going to abandon that session and take an extra day off.
- Negative thoughts are constantly holding me back. Every time I think a negative thought I'm going to immediately replace it with a positive one to change my psychology.
- I'm always going off course to the left in open water. I'm going to breathe bilaterally for 90% of every session to even out my stroke.
- Sugary drinks make my blood sugar peak and crash, making me hungry again an hour later. From now until race day, all snacks are going to be slow release.
- Nutrition is key for my channel swim next year but is something I'm not confident about. I'm going to start my research now and test thing out during my longer swims in the pool over the winter.
- My posture is terrible, I'm going to the follow the strength and posture routine in the Guru every Tuesday and Friday nights.
- From now on I'm setting my alarm 10 minutes earlier to avoid the traffic jam that makes me late for swim training.
- I'm very inconsistent with my training - boom and bust! I'm going to step back my training week by 25% but nail every single workout.
- I'm fast over 100m but underperform over an Ironman swim. I'm going to swim one Red Mist session a week from January 1st until my Ironman on 20th June. It won't be easy (at least at first) but I know it will make a huge difference.
- John makes me feel bad about myself and saps my energy, I'm going to swim by myself or find another group for the Wednesday night swim.
Each item by itself might only improve your performances by a small amount (a "marginal gain" in the jargon) while others might be worth a bigger chunk of time. Either way it's very unlikely that one single thing will make the sort of improvement in your overall performances you are looking for... but a well thought through list of 8 to 20 items almost certainly will.
Once you have written your list the rest is very simple, you just have to stick to it. Commit to the list and become the athlete you are looking to be.