I am getting fat., I want to lose weight

You want to use the noun "weight" as opposed to the verb "weigh".  Also, you have 2 choices to correct.  1.) Do as I did above, or  2.) Use a conjunction like "and" or "but" to link the sentences.  You can write as a single sentence but it sounds more natural with 2 and that way, you don't need to use a colon or have it sound a bit like a run-on sentence.

"I am getting fat and I want to lose weight, but it is hard."