"I'm happiest when I'm being myself, and I'm myself when I'm with you." - Unknown
Oftentimes, it's completely okay to give a partner the benefit of the doubt; other times, it's better to stop with the excuses. The chief issue with excuses is that it's like you're not allowed to feel what you feel, but you are. For example, if your partner cancels on a date last minute, it's okay to feel upset and ask them for more notice in the future.
On the other hand, it's not okay for you to blame yourself and make excuses for them: what if it's because I didn't confirm plans, they surely had a good reason, I'm probably overreacting, etc. It's up to you to bring to their attention how their actions make you feel, and it's up to them to apologize and ask how to be better in the future.
No matter the reason for someone's behavior, you are allowed to feel what you feel. Excuses for someone else's behavior may provide yourself with some sense of momentary comfort (e.g. I'm sure he cares even if I can't feel it) but in the long-term create confusion. For instance, if your partner is "too busy" for you, you may simply accept that excuse even if you want more than that. No, they'll make time for you if they want to make time; you are allowed to want more.
When we make excuses for a partner, we put them on a pedestal. On a basic level, the message we are giving ourselves is that they are in the right, and we are in the wrong. They are allowed to cancel or do whatever we want, and we will mentally justify that with ourselves, is what we allow to happen when we make excuses for them. Instead, it's important to acknowledge both partners as equals, both capable of right and wrong and all the in-between.
We are allowed to be frustrated, upset, annoyed, etc. And we are allowed to bring that up to a partner, instead of keeping that hidden inside, which creates a two-way problem. We fail to communicate, they fail to read our minds and change their behavior. Healthy relationships, clearly, involve compromise on both ends.
All of this is not to say that you shouldn't seek to understand a partner, not at all. Sometimes we have to cancel a date last minute, and sometimes we have busy weeks, to take from our examples. The problem happens with consistent behavior that causes us to feel bad, and a failure to both communicate and agree upon reasonable changes to make in a relationship. Don't let that be your relationship: have the courage to speak up and make the change, instead of making excuses to yourself in silence.