Whats up man? Everyone struggles with this problem sooner or later trust me, we are only human. But what I have found that helps me immensely to stay motivated is to do things that keep you in the mindset of reaching your goals on a daily basis. Some of this might sound funny to you, but here is what I did to stay motivated. And these things helped me build 15-20lbs of muscle and go from a skinny looking nerd to someone people started asking fitness advice from. Here's what I did:
1. Keep a calendar of some sort on the wall where you will see it everyday. (Next to the sink or similar area) Write what workout you are going to do the next day or two on it in big letters so that you are already in the mindset that it is in your schedule.
2. Keep a photo or a couple of photos pinned up next to the calender of things that make you want to acheive the body you want. ( I kept pics of Mila Kunis and Jessica Alba in bikinis.) Hey-it worked! I also kept a picture of a physique competitor named Steve Cook up because I wanted to look similar to how he does. While it was un-realistic that I would achieve his physique in a short term, I knew this fact and only wanted to get a similar appearance.
3. Keep Mens Health or Fitness magazines lying around the house where you will see them when you are doing daily tasks. Pick them up every so often and read an article out or just skim through the pages. I found that this not only kept me motivated, I was constantly being educated on fitness and nutrition.
4. Watch You-tube videos that make you want to go kill the gym and find some tunes that really get you pumped up!
5. Find a good pre-workout supplement. I took Lemonade C-4 for one reason: to get that tingling sensations and get my butt off of the couch and go to the gym! I dont necessarily believe pre-workout supplements provide any benefit besides this, but it is a good one!
Well I hope this helps! Good luck bro.