The boil and fit mouth-guard, once it's fitted properly, should not loose it's fit, especially not that soon. Some wear and tear can cause it to become a little loose over time. I've had mine for a couple of months and it still is well fitted to my upper teeth. Mine is one of those clear rubbery ones that cost like 2 bucks. You might want to try fitting another one as that one could be bad.
More general mouth-guard selection information:
There are two mouth-guard types, single and double mouth-guards. Single mouth-guards protect your upper teeth and double mouth-guards protect both your upper and lower teeth.
Single Mouth-guard benefits:
- Lighter weight
- Well forming
- Smaller size and profile
Double Mouth-guard benefits:
- Prevents less jaw movement
- Knockout/Concussion Mitigation*
- Decreases jaw movement
*I've heard many times that double mouth-guards help mitigate 'button' knockouts by removing additional jaw movement, but I honestly don't thinks that is completely true, just based on my medical knowledge.
Both a single and double mouth-guard will help protect your upper teeth and only a double mouth-guard will protect your lower teeth. Normally this isn't an issue as your lower teeth have a much smaller profile and are less likely to get knocked out. Both mouth-guard types will keep your teeth from slamming into each other, but the double mouth-guard will provide a little more protection in this area due to it's increased profile, decreasing the amount of vertical jaw movement.
My major issue with double mouth-guards is that they have a small breathing channel, compared to your normal unobstructed mouth. This decrease in tidal volume could pose some issues when your really sucking wind and impact air exchange. Personally a nice single mouth-guard well formed to you upper teeth will provide all the protection you need, while not decreasing your tidal volume. I use a single guard for all MMA activities and when I Airsoft. Take good care of them, cleaning them off and follow @stslavik post for changing your mouth-guard out.