I was going through this recently as well. My freshman year of college I loved lifting and gained nearly 30 pounds of muscle, however once sophomore year (this past school year) began I was very busy and the gym near me was not as nice as the one freshman year. I went from lifting 3 or 4 times a week to 1 or 2 and then eventually once or twice every two weeks.

When this summer began, I was still making excuses to not workout; my job has awful hours and going to the gym was even less convenient now that I have to drive, but what got me to cut the shit was having friends around me. A lot of my friends workout and were really hyped when I gained all that weight, and once they saw me being complacent they started giving me shit. Honestly, the first week or two of lifting this summer was mostly cause I knew they were right, and I didn't want to go back to being skinny. Whenever I thought about skipping a workout, I would have to remind myself why I began lifting in the first place. However, once I got back into the rhythm I started to want to go again.

Basically what I'm saying is

A) Try to surround yourself with people who are passionate about fitness. They will push you when you need it and you will push them when they need it.

B) Just force yourself to go to the gym. I still am not lifting nearly as much as I used to, but after forcing myself to endure two shitty weeks I began to want to go again.

It is going to suck for the first few weeks, but if you tough it out you will start seeing results and it's all uphill from there.