Pilot reported entering airspace without a clearance and later being informed that he came within 300 feet of another aircraft.

Narrative

After landing; Ground Control gave me a phone number to call for a possible pilot deviation. After engine shutdown I called the phone number given and talked to the Tower Controller at GEU. The Controller informed me that I had just flown through their airspace without a clearance and that I had come within 300 ft. of another airplane. During our conversation he asked for and I provided him with all my pertinent information. He then told me that the Tower Chief would call me if there was anything else they needed. After hanging up the phone I just sat there trying to digest what had transpired. How could I have made such a blatant mistake. I've been flying out of this airport for over 8 yrs. The airspace out here is congested. There are 4 airports within 15 miles of each and all underlying Class B airspace. Because it's rare to get a clearance into the Class B over here; it is just common practice when departing to request clearance to fly through GEU's airspace. Both going and coming. But on this day at about 30 miles out and 7;000 ft. I called Approach to request clearance into the Class B. And to my pleasant surprise my request was granted. I remember thinking this would be great! Instead of having to keep changing altitude to stay under the Bravo; I would just set up a nice smooth descent to the downwind. And since we were landing to the north I would remain high enough not need to get clearance through GEU's airspace. All of that was my thought process; about 20 miles back and 7 minutes earlier. The result was that I had allowed myself to become complacent. Something I have fought hard against my entire flying career. I did Not make sure to stay on top of my position.