Why won't my truck stay running

Why won't my truck stay running I've replaced the ignition pick up, cap and rotor on the distributor, ignition coil, i did a tune up and it ran fine for a while and I let it sit a couple days and now when I when to start it, it acted like it started but as soon as I released pressure from the key it dies. I think its the ignition switch, but it looks fine and I'm not 110% on it, any advice helps.