620 is not the be all end all of getting approved for a mortgage. Some lenders will still do FHA with less than a 620 score. What I will tell you is that the data behind your credit score is very important. All lenders look for consistency in employment, income, savings patterns, payment history, etc. Credit score is just part of the scorecard, but if you haven't demonstrated a willingness or ability to pay your bills on time and manage your finances properly you're going to have a tough time.
I would speak with a local reputable lender who is willing to sit down with you and review your qualifications and tell you what you need to do to put yourself in a position to get approved. It may turn out you're approvable right now, but if not then maybe they can help you set up a gameplan for the next 6-12 months.
Rome was not built in a day. Best of luck