Is it permissible to abandon friends whom I have relied upon in my affairs without first relying upon Allah, or must I return to them after repenting from this act?

If you relied on your friends without trusting in Allah, then you must repent. Repentance does not involve shunning or boycotting them, as the general rule is that it is impermissible to shun a Muslim for more than three days unless there is a legitimate Sharia reason. You must also dispel the misconception of a contradiction between taking means and relying on Allah, for what is required by Sharia is to combine both.