Why was Yaakov always afraid?

He may have just had a lot to be fearful of however almost every time we encounter Yaakov he is either nervous or concerned that something bad may happen. 

By the berachos with getting caught (Bereishis 27).

When leaving for Charan he is reassured by God that he'll be protected until returning and responds with "should that happen and you'll be my God / then you'll be my God etc" (ibid 28).

Upon returning and meeting Esav (ibid 32).

When Shimon and Levi destroy Shechem (ibid 34).

When the brothers are grazing the flocks at Shechem (ibid 37).

When Binyamin needs to be sent to Mitzrayim (ibid 43).

HaShem reassures him on the way down to Mitzrayim telling him that he needn't be fearful (ibid 46).

Asks sons to bury him in mearas hamachpelah even though Yosef swore to that effect already (ibid 49).

The Talmud says that the fear from Esav—despite his having been promised safe return—was due to Yaakov's fear of sin potentially undoing the prophecy (Bavli berachos 4a).

Rambam proves from there that personal prophecies for good aren't guaranteed if the person sours—as opposed to prophecies for good for a group (hakdamah to mishnah).

On the other hand Rambam asserts that Yaakov was overly fearful with regard to Esav which affected his level of prophetic perception (shemoneh perakim chapter 7).

So which is it—was Yaakov reasonably concerned for sin ruining his fortune or was he unnecessarily fearful without good cause? 

Perhaps they aren't contradictory.

If sin causes bad things to occur, and everyone sins, then logically we should always be fearful of bad things happening to us.

It must be that insofar as we try to live righteously then unless we know that we did something egregious we can reasonably proceed with confidence and optimism.

Should someone—despite living according to the Torah—be overly fearful of sin causing bad things to happen we would rightfully term such a person anxious and nervous.

The gemara is explaining how it was possible for Yaakov to have been afraid despite having been promised that it'll be okay, explaining that sin can undo a prophecy for good, with Rambam incorporating that in his introduction to the Mishnah.

In Shemoneh Perakim Rambam is focusing on why Yaakov was afraid, despite him living an exemplary life and having been given a divine promise that all would be okay, reasonably concluding that Yaakov must have been a nervous type and therefore suspected the worse.