Make the game you want to play - don't worry about pleasing other people!
This isn't to say to reject any and all critique, but don't make a game to meet the desires of others, you'll never please everyone and never finish it. Make a game that you're excited to make!
If you lose motivation, take a break and try to relax a bit, you could be over stressing yourself. If you find it hard to get back into the swing of things still, then try looking at other fangames, or other completed fan games! you may find inspiration! I just saw a whole slew of games in progressing showcase that got me excited to work on a game!
If you're having trouble figuring out where to start, start simple! Whats the point of your game? what do you want the theme to be? whose the mascot pokemon? IS there a mascot pokemon? What pokemon are you including? What is the professors name? Who will you be playing as?
Start somewhere! If you're not sure what the plot will be, then try thinking about where you want the region to take place, and how you want it to look, you could be inspired to base the villain off of a popular folk-lore or tale from the area you want to create.
If you don't know where to start a map and are getting discouraged, think of the kind of map you want to make - eg, will it be a beach map? a forest map? a mountain map? etc, and then just map!! seriously, the best and easiest way to map is to just start somewhere.
Take it from me, you're never finished with a map! You'll always look back on it and see something you want to change, so don't stress making the map "perfect" on your first go!
Spriting can be real hard, especially if you want to make a unique character! A really good way to start is to find concept art to base it off of- eg, what will the hairstyle look like, know it? google it! then try, how will their outfit look? find a reference!
A really great way to get progress done is to find someone you can brainstorm or bounce ideas off of! I have an amazing friend I talk to about fan game shit and even a short conversation with him gets me thinking of amazing ideas, and likewise with him! I've given him so many great ideas out of the blue just from a short conversation! it's really bizarre! Talking really gets your creative juices flowing, so take any opportunity you can