Luvie, it is true that some women do put on weight with progestogen-only forms of contraception - e.g. the 'mini' pill, the injection, implant or mirena. However it is not the method of contraception that makes the weight pile on, its because in some women progestogen increases appetite, thus the women eat more, thus they put on weight. As long as you are aware of this and continue to eat as you always did, it is unlikely to happen to you. As Mirena only releases a very small amount of this hormone, the chances of massive weight gain are very small, and any changes in appetite tend to settle after the first 3 months.
The loss of libido is also a risk with progestogen-only forms of contraception, but its one of those things that you just have to try the method to see if you are one of the unlucky ones who this affects, there's no other way to tell I'm afraid! But again, any side-effects do tend to settle down after the first 3 months, so your doctor is right to tell you to try and persevere. I know the bleeding is a pain but if you can try and stick with it for a bit longer you may well find it settles down in the next few weeks.
HTH