Alonso starts Sunday's Barcelona-Catalunya Grand Prix in last place, the 44-year-old outqualified for the first time in 42 races by Canadian teammate Lance Stroll.
Stroll, son of team owner Lawrence, qualified 21st and with a best lap more than four seconds slower than George Russell on pole position for Mercedes.
"We knew we have the worst car and the worst engine and we've been very clear in every race so far that we have to work," Alonso said at the Circuit de Catalunya.
"We repeat the same thing and it's exhausting. We're last, we know it, and we have no problem admitting it," he told Spanish media.
"We have the worst engine, very poor energy deployment, gearbox problems and aerodynamic problems. We're waiting for the second half of the year, and I hope we can improve a bit when the new car arrives."
Aston Martin started a new partnership with Honda this season but are making their own gearbox, with the car overseen by F1 design great and multiple title winner Adrian Newey, who moved from Red Bull.
While getting the gearbox and power unit to mesh has been one problem, the car has also had other significant issues.
The team started the campaign with a car unable to complete a race distance, drivers limited to a few laps for fear of suffering nerve damage from vibrations coming through the steering wheel.
Alonso's future remains uncertain after this season and he is unlikely to race at the Barcelona circuit again in Formula One, with the track dropping off the calendar after Sunday until 2028.
Madrid, with a new Madring circuit, will host the Spanish Grand Prix in September.
