Chinese equivalent of "I can honestly say..."
Below all are fine.

(1)坦白說,...
(2)老實說,...
(3)坦白講,...
(4)老實講,...
(5)說[老]實話...
(6)說真心話...
(7)[我]打從心底說...
(8)[我]敞開心房[向你]傾訴 (more literal, less colloquial)

+我已經盡[全]力了

or

+我已經做到最佳(表現,etc).

If you are writing a formal letter or convey in a more literal way, you may say,


我已付諸全力(+以赴).