I had the same problem with my many games at GoG, a while ago. Since I play WoW a lot, all the games started to get older there. ;)
I started a 'backlog-blog' in another gameforum and I got some 'help' from the other users there.
1. Go through your list and (if possible) put some tags on them. (next to play, done, have a look at, trash it)
2. Don't play games you don't like, just because you have them. If the game is really old, has a lot of bugs or other problems, just skip it.
3. Don't waste your time. Use walkthroughs, if you're not able to find clues in games with many puzzles (p.e. Point and click adventures)
4. If you are not sure what kind of game it is and if you would like it, watch a 'lets play' on YouTube. The same goes for very old games with bad graphics or complicated rulesets. (Old Dungeon and Dragons-games)
5. Make yourself an appointment for playing. Play 1 or 2 evenings in the week until it get's like a routine. If you like a game, you will play more often, if you have the time for it.
With these tips I reduced my backlog a lot, because there were many games I didn't like. (some were for free and/or a genre I don't like, that I didn't look at while buying some bundles).
Then I put the tags on the games and started playing with searching for a good and complete walkthrough, just in case. That helped me a lot and I played many games from my list in the last two years.
I still got many games left, but I'm a little proud of myself with what I have achieved until now.
Oh, and don't buy to many new games. ;)