A promise to your future self is an agreement whose second participant cannot negotiate yet.

You are making an agreement on behalf of someone who cannot yet argue back. The future version of you will inherit the terms without having had a say, and they will be tired, or busier, or in a mood you cannot anticipate, which is why so many of these promises are broken by the person they were made for. One party sets the terms and the other pays.

So negotiate for them in advance. Make the commitment smaller than the ambitious version of you wants, put things in place that make it easier rather than relying on resolve, and build in a way to renegotiate honestly. Treat your future self as a person you are looking after rather than one you are commanding.