Don't use relationships as an excuse to never focus on yourself, your flaws or your personal growth.
It sounds schlocky, but liking who you are is important.
It's fine to make plans with your partner and even discuss a possible future together, but it's just as important to establish for yourself what you want out of your career and work toward the things you want in your personal life.
Spend some time charting short-term and long-term goals that have nothing to do with your significant other.
If the state of your relationship entirely determines your mood, then you are probably too consumed by it. If you have plans with family or your best friends, don't flake last minute to stay in with your significant other. Someone who truly loves you and who is worth loving is secure enough to cheer you on.
It's easy to look to your partner to shelter you from the world and distract you when everything else makes you want to crawl into a hole, but continue to fight your own battles.
It's nice to have someone who wants to comfort you, and it's perfectly all right to let him or her, but make sure you don't it. You do not want to be the person who brings every conversation with friends back to the time your partner said this or did that.
Don't compromise or undermine your own desires just because a) you want to give the other person everything they want or b) you're scared that you'll lose him or her if you need something different.
A functional relationship makes room for what both of you need, and your partner can't know what you need if you don't voice it.