you are correct, soundproofing is beyond most peoples budget. i dont know how much youve got but its safe to say its probably out of budget. imagine how much it costs to acoustically treat a room, then multiply that by fuck loads 
its sad to hear you are losing interest 
i agree with both _ms and TvMcC. using good headphones or monitors at low levels are both great solutions in your position.
just because you have monitors does not man you need to be loud. i have monitors that can go loud enough to make the neighbours murder me (especially where i live), but i dont think they have ever gone past half of what they ar capable of. it is far too loud for monitoring music anyway. the ideal monitoring is much quieter than people think.
i was chatting with the woman who lives next door to me, the only barrier is a thin, cheap wall, and she assumed i listen to rap. i had lived here for about a year by then and never listened to rap. i hate it, and my music is VERY different but she had no clue, even after recreational listening at high-ish levels (certainly far beyond what i would monitor at).
that said, i obviously do not know your situation the way you do, but if you have the money and you are willing to spend it to improve your production then monitors is one of the most important factors in the whole process.
that said #2, you said you did notice a difference after running the “mastering” process, so even if you really dont want to, or cant invest in monitoring, then (going back to the original topic) you can still return to the mix and repair mistakes there to the best of your ability with the equipment you have. after that you can always send your music to a mix/mastering house to be finished there.
when my first few tracks were signed i was working on shitty headphones from the worst quality equipment i have ever had, so i sent my music to somebody else to be mixed. it was painful to let somebody else mix my music, but i knew i was in no position to do it myself with my equipment. i did not have to do this until i knew a label was interested in a track, then i would send it to the guy i was working with and he would mix it so i only had to pay for that track to be mixed.
the problem with that is trying to promote your music while it is badly mixed. the mixing is 40% of the production. its very important at that stage to keep good communication with your mix engineer throughout the process. if you can be there, he/she will hate you, but be there, so you can control the mix in the ways you need to.
another option is to hire a studio. once everything is produced you can mix fairly cheaply in a studio. before you go in make sure you are happy 100% with production (do not change your mind once you are in) and bounce everything individually, then go in there with the sole purpose of mixing. you can be in and out in a couple of hours if you focus on the task at hand and only that. that can be expensive though unless you can find a cheap studio to rent.
hint, i found a college once that was renting out their space for cheap because it was a college, not a production studio. the cost was reduced massively, but there was not really anybody around if you needed anyone. they had all the gear i needed, but i was on my own. for me that was nice though 
