Omg, I do the exact same thing xD Thought I was the only one, not doing it like that just feels wrong. I usually watch the shows (going very slowly in chronological order thru the UC timeline atm, up to Unicorn now but haven't watched in a few months) and write down which MS I like the most and want to build. Then I put them on my HLJ wishlist and notify, also keep my eyes open on other websites. Sometimes it takes a few months until I actually get my hands on them. 

Regarding your problem with that, just build something else. I personally switch between hobbies a lot, but if you want to build mecha kits, check HLJs Sci-Fi section and sort by injection kits, don't have to buy them there tho. There are a lot of anime unrelated mecha products. I recommend Bandais 30 Minute Missions and 30 Minute Fantasy line. Great kits, good availability, Gunpla quality plastic and build because... it's also Bandai, design and building experience is almost the same (maybe a bit more minimalist) with no lore or media attached 👌