@Mann Phan First, I want to compliment you on getting into real estate investing and purchasing six rental properties. I think you are experiencing what it is being an investor. From the information you have provided and don't even feel that you have done anything wrong. There might be areas for you to improve, but we all get an investment at some point that doesn't seem to work out.
I feel there are several comments saying you might not have done well with inspections, but even if you had a great inspector come out and say the HVAC was great, that doesn't mean a week after I buy the house that it may not die on me. It could even be only 3-5 years old. There are just expenses that come up. As an investor we just have to minimize our risks and plan accordingly. If the HVAC was only 3-5 years old, I probably wouldn't think it was going out and maybe not have as much cash saved for it, but if it was 20-25 years old I would think it will die soon I better be prepared.
You just need to take a look at your properties, understand the risk profile of each in terms of when certain CapEx items might need replacing, and decide if that is the risk profile you are comfortable with. If it is not, then maybe it is a good time to sell. If it is, great, because real estate investing really is a long-term investment. Don't let your emotions get the best of you because you see your cash flow going down with all these repairs. Follow your strategy and stick with it.